#2860 fix bpx data interpolant

[rtimms]

Description

Fixes a bug where all data included in a BPX was assumed to be given as a function of time.

Fixes #2914

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

[rtimms]

Sorry, got the wrong issue number, d'oh! This fixes #2914, not #2860

[codecov]

Codecov Report

Patch coverage: 94.33% and no project coverage change.

Comparison is base (7e681e2) 99.70% compared to head (0c2f416) 99.71%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2957   +/-   ##
========================================
  Coverage    99.70%   99.71%           
========================================
  Files          245      245           
  Lines        18659    18693   +34     
========================================
+ Hits         18604    18639   +35     
+ Misses          55       54    -1     

Impacted Files	Coverage Δ
pybamm/parameters/bpx.py	94.44% <94.33%> (+1.76%)	⬆️

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[brosaplanella]

Looks good, thanks!