[BioPython] handling sequence ambiguity in SeqUtils

[John Corradi]

Hi All,

I just noticed that the protein molecular weight caculation in 
ProtParam.py chokes on sequence amibiguities (i.e. X's).  It just throws 
a KeyError exception.  How about using either an average amino acid 
molecular weight or calculating a minimum and maximum for those cases?  
Thanks.

John

P.S.  I guess this is a consideration for the other utils as well.