Refactor sites methods/attrs into other classes

[stefsmeets]

This PR removes the SitesData class and moves all its methods/attributes to other classes.

Closes #240

Todo

• radial distribution function
• legacy
• update docs
• fix tests
• check if diff_trajectory is passed correctly in transitions -> it seems that jumps.n_floating returns 104 (all species) instead of 48 (Li only). This does not seem right. -> .n_floating returns the wrong number #252
• is Transitions.floating_species necessary?
• Rename/refactor tests/integration/sites_test.py
• consider renaming structure -> sites on Jumps/Transitions now that name clash with SitesData is no longer an issue

[stefsmeets]

@v1kko Could you review this one?

[v1kko]

So many movements and name changes, but overall everything seems to be in order, I left some small comments, but otherwise I think it is good to go

[stefsmeets]

There is a remaining issue that the rdf is not calculated correctly. It needs the full trajectory. When passing transitions, it just calculates RDF for 'Li'.