Allow non-numeric coordinates (e.g. time) as spatial axes

[annehaley]

This PR enables selecting coordinates with non-numeric dtypes for spatial axes. Since the true values of a non-numeric coordinate cannot be used on the mesh, a representative value list is created to use on the mesh. This representative list is similar to an index list, but it is scaled according to the step value used to slice that coordinate.

For example, if a coordinate time is selected for the Z axis and a slicing of [0, 20, 4] is applied to time, then mesh.z will have the following values: [0, 4, 8, 12, 16] .

In order to be compatible with non-numeric coordinates, this PR also reorganizes how slicing is done. Expected values for slice arrays must refer to indexes rather than values in the coordinate array.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 43.09%. Comparing base (dceabc7) to head (c1b9ef9).

Files	Patch %	Lines
pvxarray/accessor.py	62.50%	3 Missing :warning:
pvxarray/rectilinear.py	75.00%	1 Missing :warning:
pvxarray/structured.py	80.00%	1 Missing :warning:
pvxarray/vtk_source.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #65 +/- ## ========================================== + Coverage 40.12% 43.09% +2.97% ========================================== Files 11 11 Lines 481 478 -3 ========================================== + Hits 193 206 +13 + Misses 288 272 -16 ```

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