Fix bug in getting experimental sequence

I should test stuff more.
Use correct chain identifier
Add a parser for dssr style ids

We can now attempt to parse dssr style ids correctly. Note that this is not
fool proof as dssr style ids can be ambiguous in cases of modified bases. These
bases may have names that make it hard to determine where the number portion of
the id begins.
Add a simple infer option to dssr parsing

We can attempt to infer the residue name by looking for the last letter in that
part. This is flawed but will work for many residue names. I've added such an
option now.