Refactor quick_FASTA_reader (shorter like this)
Fix import to work on Python 2 and 3
Fix an old style list.sort()
Refactor this use of a sorted list
This should probably raise the NexusError (otherwise catch & reraise is silly)
PEP8 whitespace tidying in Nexus.py
Don't need to deep copy dicts, nor plain strings.

We do still need to avoid letting Bio.Nexus edit the
global alphabet instances however... but the deepcopy
is not the best solution for that.