Redesign of the PreconCfs class and subclasses

Issue/feature addressed

The subclass PreconCfsOp seems to be useless. This MR propose a redesign of the PreconCfs class and subclass.

Proposed solution

• Makes virtual function in the base class PreconCfs pure virtual
• Delete PreconCfsOp subclass
• Make PreconCfsBRJ subclass inherit directly from PreconCfs

Implementation

A more detailed description of the changes. Please include any details necessary for reviewers to understand your work, and point out particular points would like them to pay particular attention to.

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).