Should we have a JuLIPData package

[cortner]

CC @jamesgardner1421

Maybe we should avoid keeping any data files in JuLIP, but load then all into a JuLIPData package. The next step might be to use that package to automatically download a large number of potential definition files. We experimented with this in our tightbinding code where it worked really well.

[jamesgardner1421]

That seems sensible. It's not very neat to have them in here.