Tidy and clarify implementation of Dirichlet boundary condition in GlobalLinSys

Issue/feature addressed

This MR has the purpose of clarifying the implementation of the Dirichlet boundary condition in GlobalLinSys

We are basically solving

A * x = b 
A * (xprev + xdirichlet + deltax) = b
A * deltax = b - A * (xprev + xdirichlet) = rhs

x = (xprev + xdirichlet) + deltax

This MR propose some variable name and comment changes to clarify the implementation. Also, it uniformizes the implementation across

• library/MultiRegions/GlobalLinSysDirectFull.cpp
• library/MultiRegions/GlobalLinSysIterativeFull.cpp
• library/MultiRegions/GlobalLinSysPETScFull.cpp
• library/MultiRegions/GlobalLinSysXxtFull.cpp

Remark: Due to a previous misinterpretation, the following is currently implemented in the redesign project

A * (xdirichlet + deltax) = b 
A * deltax = b - A * xdirichlet = rhs

x = xdirichlet + deltax

Proposed solution

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).