Tidy-up: Rename VCSImplicit formulations for advection velocity

Issue/feature addressed

The implicit Velocity-Correction scheme allows two choices of advection velocities for the linearised operator, however, current naming is non-intuitive.

Proposed solution

Renamed from original author to intuitive names that relate to the numerics of the different forms.

Implementation

Tests

Suggested reviewers

@mlahooti

Notes

• Changes are inline with updated IncNavierStokesSolver documentation

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).