Added helper function in DomainBox to figure out if any boundary is "special"
Renamed SerenError to GandalfError and instructed ExceptionHandler for MPI
Made Ghosts an abstract class, created children classes and modified the code accordingly
Created particle MPI datatype
Added check that MPI and OpenMP can work together for hybrid parallelization
Merge branch 'mpi' of github.com:gandalfcode/gandalf into mpi

Conflicts:
	src/MpiControl.cpp
	src/gandalf.cpp