Add option to download structures from PDB

[ChrisWellsWood]

I imagine that lots of people will test it, or include, structures from the PDB. As all the metrics have been precalculated and cached for these structures, it makes sense to have a separate input method for these so that we don't recompute them every time. I don't think this is super high priority, so I'm not putting it into the v0.1.0 milestone.