Issue 2670 interp2d

[valentinsulzer]

Description

Fixes the interp2d warning by switching to using xarray as the backend for ProcessedVariable

Fixes #2760
Fixes #2366

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all
• The documentation builds: $ python run-tests.py --doctest

You can run unit and doctests together at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

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Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (9b4490b) 99.69% compared to head (9ebbb11) 99.68%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2907      +/-   ##
===========================================
- Coverage    99.69%   99.68%   -0.01%     
===========================================
  Files          273      273              
  Lines        19086    19009      -77     
===========================================
- Hits         19027    18950      -77     
  Misses          59       59              

Impacted Files	Coverage Δ
pybamm/solvers/processed_variable.py	100.00% <100.00%> (ø)

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[brosaplanella]

Looks good, thanks!