search for self-build mpirun in generic.run

[Roland Haas]

If no MPI stack is found ExternalLibraries/MPI will build its own MPI library and runtime tools and install eg the tools into `exe/sim/mpirun` which is the expected location for utilities build by thorns.

However generic.run does not use this mpirun meaning that in situations where MPI is not present (ie someone’s laptop) one has to manually edit RunScript

Pull request:

https://bitbucket.org/simfactory/simfactory2/pull-requests/46/generic-search-for-self-build-mpirun

adds exe/sim to PATH in generic.run so that, if present, mpirun from there is used.

[Roland Haas]

Approved by @Steven R. Brandt . Thank you!

[Roland Haas]

Applied as git hash 031b87a "generic: search for self-build mpirun" of simfactory2

[Roland Haas]

This breaks on the Jenkins serer since mpirun assumes that it can find other MPI tools (hydra_pmi_proxy specifically) in the same directory it itself resides.

Reverting until a better solution is available.