Bump to 0.1.0
Merge branch 'develop'
Bump to 0.1.1
Update ignore
Improved polymer detection

This way we find polymers by using the pdbx_unobs data which tells which
polymers are not observed. This is better than trying to infer it as I was
before. In addition, I've added several classes to represent things like models,
chains, and residues. This improves the design by allowing us to iterate over
parts of the structure to find polymers and generate unit ids.
Add residue/polymer methods

Add methods to look up a residue or polymer by index. This is useful, but
currently requires iterating over all polymers/residues each time. This is not
good and will be fixed later by memoizing the list of residues/polymers.
Correct mistakes in tests