Representation theory and equivariance notes

Created by: attila-i-szabo

As promised at #658 (closed), I wrote some notes on representation theory in space groups.

• covers the terminology as accessibly as I could do it
• gives an overview of the construction of the character table (or rather, specific irreps, which is what we need to pick out symmetry sectors)
• turns the construction into an algorithm that depends on geometric manipulations as little as possible
• discusses how to hook this up with equivariant nets (given the complications around #698 (closed), I'm thinking of splitting part of this into a separate note that elaborates on how equivariance works)

I didn't yet hook it up with the website, because there weren't any headings in the documentation where a note on maths would really fit in...

@chrisrothUT @PhilipVinc @femtobit