[WIP] Full multigrid with implicit line agglomeration

[bigfooted]

Proposed Changes

Give a brief overview of your contribution here in a few sentences.

• fix full multigrid. FMG now has scaled CFL for the coarser meshes during the warmup phase. We also have
  MG_STARTUP_ITER= 50 as config option.
• fix implicit line agglomeration. for the turbulent flat plate and single core computations, we get an exact agglomeration. new config options:
  MG_IMPLICIT_LINES= YES
  MG_IMPLICIT_LINES_MAX_LENGTH= 1000
  MG_IMPLICIT_LINES_ISOTROPIC= YES

For isotropic=yes, every pair of 2x2 nodes is agglomerated, for isotropic=no, we only agglomerate in the direction normal to the implicit line, so the y+ values are preserved.
Note that this implicit line agglomeration is a 2D implementation.

Related Work

PR Checklist

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