MPI option for codes

[unkcpz]

with_mpi option can be set, verdi code create core.code.installed -h shows:

--with-mpi / --no-with-mpi      Whether the executable should be run as an
                                  MPI program. This option can be left
                                  unspecified in which case `None` will be set
                                  and it is left up to the calculation job
                                  plugin or inputs whether to run with MPI.

Makes a lot sense to explicitly set this option for all codes in cscs and merlin.