Rearrange the HPC section

[alasdairwilson]

          I am reviewing this in detail but I initially had just a general thought on the layout, I think the high performance computing theme is organised incorrectly. 

The "Theme" page has too much specificity in it, take for comparison the technology and tooling theme where one course is on say "IDEs" and another on "Testing" to the HPC theme where one course is on "Parallel Computing" and another is on "Introduction to parallelism" and another on "Introduction to supercomputing". I know this is not all added in this PR though.

There would still need to be work to do to get the rest of the Theme in line and these 4 "streams" could be linked together in the theme diagram too which would not be represented in the default gridbox layout.

For example openmp and MPI would be merged into a single "course" called something like "parallel computing tooling" or "parallel computing libraries" with the openmp and mpi as separate streams.

Originally posted by @alasdairwilson in #187 (review)