[multigrid_gravity] Removed local FFT solver as it is dead for too long
Removed prefer_rbgs_relaxation from various problem.par
[multigrid_gravity] removed prefer_rbgs_relaxation parameter
[multigrid_gravity] enabled FFT solver only on the base level
[multigrid_gravity] removed code obsoleted by c30e22dfcede8e0c09b69f251df4bad1b9a7a525
[multigrid_gravity] Improved sanity checks for FFT base level solver
Merge branch 'MG_base_fft' into int32