Cleanup and restructure parallel solver helpers
The following discussion from !1839 (closed) should be addressed:
@blattms Do I see it right that all solver use the same parallel setup as the AMG? So amgtraits.hh and amgparallelhelpers.hh are not specific to AMG? Then we could just rename the headers and AMGTraits->ParallelSolverTraits so it's less confusing.