Openfoam expression
This adds an OpenFOAM nix expression. It
* provides binary wrappers that automatically source the *etc/bashrc* script before running, and
* provides *-mpi wrappers that automatically call the correct mpirun with `-n <num>` set by the partition.
It has been tested on goblin in both MPI and non-MPI mode on the [damBreak example](http://cfd.direct/openfoam/user-guide/damBreak).
See merge request !1
