updated .gitignore

set up .gitattributes for selected files

[bin] set all scripts executable

[multigrid_gravity_helper] fixed messages

[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