Added configuration option and associated documentation for setting up MPItrampoline

[fergu]

This is a small change to allow a user to define JUHPC_MPI_TRAMPOLINE to have MPIPreferences configured to use MPItrampoline per the instructions here.

This will fix an issue where, if system versions of both MPI and HDF5 are specified, julia will segfault during precompilation of HDF5. This is a known issue associated with calls to MPI from HDF5 incorrectly picking the julia-supplied version of MPI, even when MPIPreferences is pointed to a system version. See JuliaIO/HDF5.jl#1079 and JuliaParallel/MPI.jl#835. The recommended workaround for this is to use MPITrampoline: JuliaIO/HDF5.jl#1079. There's been some interest in making MPITrampoline the default, so supporting this is probably good: JuliaParallel/MPI.jl#801

The main downside to this approach is that MPItrampoline requires the user to compile and install a wrapper and set an associated environment variable: https://github.com/eschnett/MPIwrapper, which is an extra step but not terribly difficult.

The changes are:

README.md: Adds a description of JUHPC_MPI_TRAMPOLINE and modifies the descriptions of JUHPC_MPI_HOME and JUHPC_MPI_VENDOR to indicate that JUHPC_MPI_TRAMPOLINE is incompatible with the others.

juhpc: Modifies line 218 to install MPIPreferences if JUHPC_MPI_TRAMPOLINE is defined. Also adds an additional elif statement that is executed if JUHPC_MPI_TRAMPOLINE is defined (and JUHPC_MPI_HOME and JUHPC_MPI_VENDOR are not). This sets MPIPreferences to use MPITrampoline_jll per the directions here.

[omlins]

Nice! Thanks for the detailed explanation of your PR. If you ever feel like going one step further and automatize also the MPI trampoline installation and environment variable setting, then please open the PR again. In any case I'm merging it now as is.

[omlins]

@fergu : released in v0.6.0!