Bug fix GroupedSPMe to enable experiments

[NicolaCourtier]

Description

Introduces separate variable names for the internal and output variables representing the particle surface voltages. Many thanks to @noelhallemans for spotting this and @martinjrobins for debugging it!

Issue reference

Fix following #584

Review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.22%. Comparing base (c8b00e6) to head (c2eef53).
Report is 37 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #592      +/-   ##
===========================================
+ Coverage    99.08%   99.22%   +0.13%     
===========================================
  Files           52       57       +5     
  Lines         3605     4240     +635     
===========================================
+ Hits          3572     4207     +635     
  Misses          33       33              

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