Added MPI based implementation of multiple walkers

It is still preliminary and not very well optimized.
It only works with MPI (i.e. all processes should be part of the same
MPI job). In the future we might think to allow groups of MPI walkers
exchanging information through the filesystems.

Addresses issue plumed/plumed2#12
Added regtest for WALKERS_MPI
Merge branch 'master' into walkers-mpi
Merge branch 'master' into walkers-mpi
Merge branch 'master' into walkers-mpi
Merge branch 'master' into walkers-mpi
Merge branch 'master' into walkers-mpi
Added comments
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