Overview:
  - `xcdat` APIs previously expected a 1:1 map between axis dimensions and coordinates. For example, the `time` axis dimension would be mapped to `time` coord var. However, datasets can have N dimensions mapped to N coordinate variables
  - For example, datasets that have N dims mapped to N coord vars include E3SM, which might have a "Z" axis with the dimensions and coords "ilev" and "lev"

Feature Updates:
  - Update bounds accessor methods to operate on the dataset or a variable within the dataset using the new `var_key` kwarg
  - Update `add_missing_bounds()` to loop over all axes and their coordinate vars and attempt to add bounds for each coordinate var if they do not exist
  - Update decoding function (`decode_time()`) to decode CF and non-CF compliant time coordiantes as `cftime` objects
  - Update dataset longitude swapping function (`swap_lon_axis()`) to loop over longitude coords and their bounds to swap them
- Update spatial and temporal accessor class methods to refer to the dimension coordinate variable on the `data_var` being operated on, rather than the parent dataset

Bug Fixes:
  - Fix `add_bounds()` not ignoring 0-dim singleton coords in addition to 1-dim singleton coords

Refactor:
  - Refactor `open_dataset()` and `open_mfdataset()` to remove unnecessary preprocessing functions
  - Rename `get_axis_coord()` to `get_dim_coords()` and `get_axis_dim()` to `get_dim_keys()`
  - Update fixtures in `fixtures.py` to use `cftime` objects and add `decode_times` to `generate_dataset()`
  - Extract utility function `_if_multidim_dask_array_then_load()` and reuse in several places in the codebase, usually when manipulating bounds

- This improves runtime performance, since `.values` performs type conversions to `np.array()`
- Refactor `_get_cftime_coords()` to use `xr.coding.times.decode_cf_datetime()` for decoding CF compliant time units

- Update `_is_decoded()` to check `.encoding` attributes rather than the object types in the array for performance purposes, since accessing `.values` can involve type conversion

- Refactor imports and fix docstrings

- This is required for cases where time units are different between files in multi-file datasets, but the offsets are the same. More info in the `_preprocess()` docstring
- Add raise ValueError for decoding datasets with time coords without CF attrs set

- Add `decode_time()` test for units in days

to interpret common var names
- Update tests