PseudoDojo as git submodules

[azadoks]

The PseudoDojo guys are working/have finished putting their pseudos on github, e.g. in the repo for the scalar-relativistic PBE family (there are many such repos).

We could add these as git submodules so that we don't have to host the files here and so we can avoid having stale copies lying around.

I could even ask the SSSP devs (i.e. Jusong) to do the same with their families. Slower moving things like GBRV, SG-15, and HGH can probably be safely stored here.