Dependency issues in updated MPI makefile

[ageorgou]

The makefile of the updated MPI version (currently in merge-develop-mpi) has a couple of small issues:

• Some redundant dependencies, such as for moltype.o
• mpi_aux.o is not listed as a direct dependency of tran.o and perturbation.o, which makes a serial build fail (although building in multiple jobs seems to work)