Various cosmetical changes

[multigrid_gravity] removed shortcut for clarity

[multigrid_gravity] Print the solver type only for the coarsest level

[multigrid_gravity] removed trust_fft_solution parameter

[multigrid] corrected a message

[multigrid_gravity] comment

[pcg] doxy, comments

[qa.py] disabled check for SVN Id

[qa] cosmetics

[gravity] removed unused imports