write to raster using windowed writing & add isel_window method

[snowman2]

• Closes Add ability to get raster subset with a rasterio Window #22 (start of Use windowed writing in to_raster #9)
• Tests added
• Fully documented, including whats-new.rst for all changes and api.rst for new API

The thing that I am on the fence about is if we want to make windowed writing the default or the previous behavior the default. The windowed writing can be more memory efficient, but may not be as fast reading the entire chunk into memory. There are different scenarios. So, maybe add a windowed=True kwarg to turn on/off this behavior.

Side note: since the block sizes of the destination raster are created with rasterio, it should be constant across all bands, so that isn't something to worry about.

[snowman2]

For reference (non-tiled/ striped raster):

It seems like it goes along with the stripes:

>>> rds.is_tiled
False
>>> rds.block_shapes
[(7, 574)]
>>> rds.shape
(245, 574)

So, in this case it writes out the stripes.

>>> list(rds.block_windows(1))
[((0, 0), Window(col_off=0, row_off=0, width=574, height=7)), ((1, 0), Window(col_off=0, row_off=7, width=574, height=7)), ((2, 0), Window(col_off=0, row_off=14, width=574, height=7)), ((3, 0), Window(col_off=0, row_off=21, width=574, height=7)), ((4, 0), Window(col_off=0, row_off=28, width=574, height=7)), ((5, 0), Window(col_off=0, row_off=35, width=574, height=7)), 
...

[snowman2]

Also, added fix for:

DeprecationWarning: Right multiplication will be prohibited in version 3.0
    x, _ = (np.arange(nx) + 0.5, np.zeros(nx) + 0.5) * transform