Add BPX support

[NicolaCourtier]

Description

Add support for importing BPX JSON files as parameter sets.

Issue reference

Adds feature request #79.

Type of change

• New Feature: A non-breaking change that adds new functionality.

[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 95.42%. Comparing base (9dbf1fa) to head (3a10ea7).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
pybop/models/empirical/ecm.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #246      +/-   ##
===========================================
+ Coverage    95.35%   95.42%   +0.07%     
===========================================
  Files           36       36              
  Lines         1979     1990      +11     
===========================================
+ Hits          1887     1899      +12     
+ Misses          92       91       -1     

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[BradyPlanden]

Thanks for the addition @NicolaCourtier, on a whole this looks great! I've made two small comments that are worth investigating before merging. Let me know what you think.

[BradyPlanden]

Thank @NicolaCourtier.

LGTM, just one suggestion on the unit tests.