Add interface for variable coefficients to Collections

Issue/feature addressed

Variable coefficients (varcoeffs) can currently not be given to Collections-type operators for grouped processing and, also, the GetInputSize (and GetOutputSize) routine for collections requires a boolean to return the input size in PhysSpace or CoeffSpace which is necessary for the varcoeffs.

Proposed solution

This branch adds varcoeffs to via a UpdateVarcoeffs routine which needs to be re-implemented in each operator. Since mostly the Helmholtz operator requires varcoeffs this does not require changes in all other operators.

Implementation

• Renamed CheckFactors to UpdateFactors because it reflects the action better
• Added UpdateVarcoeffs as routine in the Collections class and a virtual routine in the Operator class

Tests

Most previous tests that use varcoeffs. In particular the Helmholtz3D tests with variable diffusion

Suggested reviewers

@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).