BUG: Normal vectors in SlicePlot doesn't understand non-cartesian coordinate systems

[neutrinoceros]

Bug report

Good news, this one already has a fix waiting in #3489, but I'm reporting it independently because it wasn't my main motivation in that PR and I actually encountered this bug now.

Bug summary
because SlicePlot instantiates OffAxisSlicePlot as well as AxisAlignedSlicePlot classes based on the value of the normal argument, it implements some magic to transcribe special values of normal vectors into strings, namely:

(1, 0, 0) -> "x"
(0, 1, 0) -> "y"
(0, 0, 1) -> "z"

The problem being that this correspondence is hard coded and doesn't account for the relevant geometry, so it only works with cartesian coordinates.

Code for reproduction

import yt

ds = yt.load_sample("bw_spherical_2d")
yt.SlicePlot(ds, (0, 0, 1), "density")

Actual outcome

Traceback

╭─────────────────────────────── Traceback (most recent call last) ────────────────────────────────╮
│ <ipython-input-7-f6eeb60e5811>:1 in <module>                                                     │
│                                                                                                  │
│ /Users/robcleme/dev/yt-project/yt/yt/visualization/plot_window.py:2484 in SlicePlot              │
│                                                                                                  │
│   2481 │   │   │   )                                                                             │
│   2482 │   │   │   del kwargs["north_vector"]                                                    │
│   2483 │   │                                                                                     │
│ ❱ 2484 │   │   return AxisAlignedSlicePlot(ds, normal, fields, *args, **kwargs)                  │
│   2485                                                                                           │
│   2486                                                                                           │
│   2487 def plot_2d(                                                                              │
│                                                                                                  │
│ /Users/robcleme/dev/yt-project/yt/yt/visualization/plot_window.py:1584 in __init__               │
│                                                                                                  │
│   1581 │   ):                                                                                    │
│   1582 │   │   # this will handle time series data and controllers                               │
│   1583 │   │   axis = fix_axis(axis, ds)                                                         │
│ ❱ 1584 │   │   (bounds, center, display_center) = get_window_parameters(                         │
│   1585 │   │   │   axis, center, width, ds                                                       │
│   1586 │   │   )                                                                                 │
│   1587 │   │   if field_parameters is None:                                                      │
│                                                                                                  │
│ /Users/robcleme/dev/yt-project/yt/yt/visualization/plot_window.py:77 in get_window_parameters    │
│                                                                                                  │
│     74                                                                                           │
│     75                                                                                           │
│     76 def get_window_parameters(axis, center, width, ds):                                       │
│ ❱   77 │   width = ds.coordinates.sanitize_width(axis, width, None)                              │
│     78 │   center, display_center = ds.coordinates.sanitize_center(center, axis)                 │
│     79 │   xax = ds.coordinates.x_axis[axis]                                                     │
│     80 │   yax = ds.coordinates.y_axis[axis]                                                     │
│                                                                                                  │
│ /Users/robcleme/dev/yt-project/yt/yt/geometry/coordinates/spherical_coordinates.py:304 in        │
│ sanitize_width                                                                                   │
│                                                                                                  │
│   301 │   │   return center, display_center                                                      │
│   302 │                                                                                          │
│   303 │   def sanitize_width(self, axis, width, depth):                                          │
│ ❱ 304 │   │   name = self.axis_name[axis]                                                        │
│   305 │   │   if width is not None:                                                              │
│   306 │   │   │   width = super().sanitize_width(axis, width, depth)                             │
│   307 │   │   elif name == "r":                                                                  │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
KeyError: 'z'

Expected outcome

Should be equivalent to

yt.SlicePlot(ds, "phi", "density")

[matthewturk]

We have long punted on properly addressing this, but the recent changes to the API for slice plots (esp re:Normal vectors) make this much more feasible.