Make MPI work

Fixes #16 (closed)

For now we only add MPICH, as I need to start launching MPI tests for ovni.

Although, we still need to copy the closures to each node manually:

hut$ nix shell jungle#hut.bsc.osumb
hut$ nix copy --to ssh://owl1 $(which osu_bw)
hut$ nix copy --to ssh://owl2 $(which osu_bw)
hut$ srun -N2 osu_bw
# OSU MPI Bandwidth Test v7.1
# Size      Bandwidth (MB/s)
# Datatype: MPI_CHAR.
1                       1.30
2                       2.61
4                       5.23
8                      10.45
16                     20.62
32                     41.07
64                     82.23
128                   158.42
256                   315.83
512                   622.96
1024                 1183.65
2048                 2062.81
4096                 3090.81
8192                 4190.01
16384                4826.58
32768                5583.63
65536                7160.62
131072              10715.51
262144              11915.88
524288              12216.00
1048576             12303.94
2097152             12347.91
4194304             12384.89

We should have a shared nix store in the compute machines.