Dear Maintainer,

Running mb-mpi spawn instances of MrBayes instead of running the version
with parallel support. Compiling the parallel version from git source
produces a working version (https://github.com/NBISweden/MrBayes,
./configure --with-mpi=yes), but compiling the source debian package
doesn't.

A smaller problem is that the package is undocumented. The official
instructions say that the parallel version should be run with `mpirun`:
`mpirun -np 4 mb file.nex`. The debian package ships `mb-mpi` executable,
which is a bash wrapper that does that with environmental variable
MPI_OPTIONS passed to mpirun as options. But to figure it out, a user
needs to read the script.