MPI support for HDF5 and SGE support for OpenMPI

### MPI

I added support for the Sun Grid Engine to openmpi. And, a new package `mpi4py`, which is a python interface to mpi.

### HDF5

I added mpi support to hdf5 (by default through openmpi). And, a new package `h5py`, which is a pythonic interface to hdf5 that also supports parallel hdf5.

In the [h5py derivation](https://github.com/aherrmann/nixpkgs/blob/1e0605738a94d834c97d6e7b9d1fc920f649facb/pkgs/development/python-modules/h5py/default.nix#L5-L10) I'm checking if both `hdf5`, and `h5py` have mpi support activated, and if so, if the mpi version is the same. Please let me know if this is the proper way to do these types of checks.

Let me know, if there is something that you would prefer to be done in a different way. I'll be happy to adjust it and learn the proper Nix way.
Why do you need to override python install phase? Also why is this not in `python-packages.nix`?