[Biopython] Adding method to remove gaps in MultipleSeqAlignment

[Travis Wrightsman]

Dear Biopython Devs,

I have code that I wrote to generate a new MultipleSeqAlignment that has 
columns removed if they contained gaps in the first sequence. If I 
generalized this code to return a MultipleSeqAlignment with gaps removed 
in any specified record row, would this be something Biopython would want 
included?

I also see the Alignment class looks to be the new, more general way to 
work with alignments in 1.80, should I implement this on 
MultipleSeqAlignment or Alignment?

Travis