Volume fractions and porosity that vary in y and z

[DrSOKane]

Added support for porosity and active material volume fractions that vary in y and z, not just x. It appears to work for the z coordinate ("dimensionality": 1) but the y coordinate appears to get replaced with the StateVector between experiment steps. I tried changing all instances of y in lithium_ion_parameters.py to y_cc but that didn't change anything.

Added an example graded_pouch.py to show that it works in one dimension.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.77%. Comparing base (6ba2cde) to head (f6b8858).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5214   +/-   ##
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  Coverage    98.77%   98.77%           
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  Files          321      321           
  Lines        27727    27733    +6     
========================================
+ Hits         27387    27393    +6     
  Misses         340      340           

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