move parallel headers to common subdirectory parallel

dune/common/collectivecommunication.hh

dune/common/mpicollectivecommunication.hh

dune/common/mpiguard.hh

dune/common/mpihelper.hh

dune/common/mpitraits.hh

dune/common/parallel/Makefile.am

dune/common/parallel/collectivecommunication.hh

dune/common/parallel/mpicollectivecommunication.hh

dune/common/parallel/mpiguard.hh

dune/common/parallel/mpihelper.hh

dune/common/parallel/mpitraits.hh

dune/common/parallel/plocalindex.hh

dune/common/parallel/remoteindices.hh

dune/common/test/lrutest.cc

dune/common/test/mpicollectivecommunication.cc

dune/common/test/mpiguardtest.cc

dune/common/test/mpihelpertest.cc