Add VolumetricEnergyDensity example

[NicolaCourtier]

• add VolumetricEnergyDensity
• add cell_volume function
• extend tests
• update Changelog

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d3fa97f) 94.55% compared to head (1499994) 94.67%.
Report is 1 commits behind head on develop.

Files	Patch %	Lines
pybop/models/base_model.py	57.14%	3 Missing ⚠️
examples/scripts/spme_max_energy.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #191      +/-   ##
===========================================
+ Coverage    94.55%   94.67%   +0.11%     
===========================================
  Files           42       44       +2     
  Lines         1783     1823      +40     
===========================================
+ Hits          1686     1726      +40     
  Misses          97       97              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[BradyPlanden]

One spot where we should probably update the tests to match the intended call structure. Otherwise, nice one - looks great!