Improve error handling.

We now only translate the MissingColumn and MissinBlock into the correct types
when using __getitem__ this will prevent us from treating other exceptions
which may be raised as a sign of accessing something missing.
Improve polymer tests.

This now tests that the sequence extracted is correct. This test does use
BioPython to translate the 3 letter codes.
Bug fix.

This makes sure we actually get the polymers correctly. We had a bug before.
Remove unneeded pprint