[WIP] Feature fullmg

[bigfooted]

Proposed Changes

Fixes some small things in full multigrid method. New config option MG_STARTUP_ITER=100 for the number of iterations in the initial phase of the FMG method
Fixes some small things in the implicit line agglomeration for inflation layers. MG_IMPLICIT_LINES_ISOTROPIC=YES will create a coarser mesh that is 2x2 smaller in both x and y-directions and =NO will only agglomerate in the direction normal to the implicit line. the implicit line is normal to the wall.

Related Work

Resolve any issues (bug fix or feature request), note any related PRs, or mention interactions with the work of others, if any.

PR Checklist

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• I have added a test case that demonstrates my contribution, if necessary.
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