Add mpi support to dials.stills_process.
Add mpi support to dials.stills_process.

Mainly useful for support of experiments at LCLS outputting .mccd files, but could be used elsewhere with a lot of single images on a cluster with MPI support.

Examples:
mpirun -n 10 dials.stills_process params.phil mp.method=mpi *.mccd
Submit at LCLS:
bsub -n 64 -q psanaq -o log.out mpirun dials.stills_process params.phil mp.method=mpi *.mccd

dials/dials,dials/dials,dials/dials,dials/dials,dials/dials