Faster matrix (re)-building for LinearADR problems

Issue/feature addressed

Currently, a LinearADR problem always rebuilds elemental Mass and Laplacian matrices. This causes an overhead of about 30% for VCSImplicit simulations at large scale and is only required if Mass and Laplacian matrices are time-dependent through varcoeffs.

Proposed solution

This fix adds a check for conditional updating of Mass and Laplacian matrices to the LinearADR option in the CreateMatrix function for Expansion2D and Expansion3D.

Implementation

An if condition that checks whether the massVarcoeffs or laplacianVarcoeffs map is empty.

Tests

• We would need to have a memory-constraint test environment. Still, I am running this fix on large-scale simulations and do not encounter any memory issues.

Suggested reviewers

@mlahooti @CFD-Xing

Notes

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).