Tidy up diffusion class and subclasses

Issue/feature addressed

Tidy-up of the diffusion class and subclasses.

Proposed solution

• v_InitObject has been made pure virtual
• v_Diffuse has been made pure virtual
• Remove unused virtual function v_GetPrimVar along with public accessor
• Remove duplicate (unused) virtual function v_DiffuseCoeffs along with public accessor
• Remove unused function GetPenaltyFactorConst
• Remove unused function GetDivCurl
• Remove unused function AddSymmFluxIntegralToOffDiag
• Virtual function v_ConsVarAveJump is specific to DiffusionIP. Removed from the base class along with public accessor. Function made non-virtual in the subclass.
• Virtual function v_DiffuseTraceSymmFlux is specific to DiffusionIP. Removed from the base class along with public accessor. Function made non-virtual in the subclass.
• Virtual function v_AddDiffusionSymmFluxToCoeff is specific to DiffusionIP. Removed from the base class along with public accessor. Function made non-virtual in the subclass.
• Virtual function v_AddDiffusionSymmFluxToPhys is specific to DiffusionIP. Removed from the base class along with public accessor. Function made non-virtual in the subclass.
• Relevant member variables and/or functions are made protected or private
• Remove commented/unused code

Implementation

Tests

Suggested reviewers

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Notes

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Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• Changelog is updated.
• [ ] Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• [ ] License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).