Reading/writing orbitals to/from file

[jagot]

WIP, still remains to read/write spin-orbitals and configurations
from/to files, as well as reading from/writing to text files

[jagot]

I think I'm happy with this now; it is not possible to read orbitals or configurations directly from a text file without specifying which kind we expect, i.e. we don't have a magic method read_orbital(s::AbstractString), but rather one has to explicitly parse(Orbital, s) or parse(SpinConfiguration{SpinOrbital{RelativisticOrbital}}, s).