Manage coordinates as tuples

[michaelosthege]

Model coordinates should be immutable objects, so tuples are prime candidates for that.

Unfortunately tuples cause problems when passed to xarray (#5043) which happens after the compute-intensive MCMC.

This PR modifies the Model.coords such that all coordinate values become tuples.
This way they're immutable and one can do model.coords["city"].index("London") to obtain indices that can be used for slicing tensors.

In the InferenceData conversion tuples are then automatically converted to NumPy arrays to avoid the problems with xarray.

Note that while this PR restricts the Model.coords to tuples, the user may still manually pass other types such as MultiIndex for the conversion.

Depending on what your PR does, here are a few things you might want to address in the description:

• what are the (breaking) changes that this PR makes? → above
• important background, or details about the implementation → above
• are the changes—especially new features—covered by tests and docstrings? → yes
• linting/style checks have been run → yes

[codecov]

Codecov Report

Merging #5061 (4034162) into main (595e164) will decrease coverage by 11.97%.
The diff coverage is 94.44%.

❗ Current head 4034162 differs from pull request most recent head c9c07bb. Consider uploading reports for the commit c9c07bb to get more accurate results

@@             Coverage Diff             @@
##             main    #5061       +/-   ##
===========================================
- Coverage   78.43%   66.45%   -11.98%     
===========================================
  Files         130      130               
  Lines       24464    24441       -23     
===========================================
- Hits        19188    16243     -2945     
- Misses       5276     8198     +2922     

Impacted Files	Coverage Δ
pymc/model.py	82.75% <75.00%> (-1.35%)	⬇️
pymc/backends/arviz.py	89.35% <100.00%> (-0.09%)	⬇️
pymc/tests/test_idata_conversion.py	96.31% <100.00%> (+0.01%)	⬆️
pymc/tests/test_bart.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/test_math.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/test_moment.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/test_shared.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/test_logprob.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/test_missing.py	0.00% <0.00%> (-100.00%)	⬇️
pymc/tests/test_tracetab.py	0.00% <0.00%> (-100.00%)	⬇️
... and 35 more