Updating documentation and adding missing figures in mpi-dev doc
change name of op_write_hdf5 to op_dump_to_hdf5 and changed related documentation for clarity
Adding GPU direct execution instructions to user documentation
Updating documentation and related source files to match
Updating partitioning section
Adding some details on inheriting partitioning from the primary set
Minor update
Formatting modifications
Formatting update
Merge pull request #61 from OP2/documentation-update

Documentation update

This pull request mainly updates the MPI documentation including two
diagrams that was previously missing to build the pdf from latex source.