VAMP-seq with replicates

[afrubin]

When performing an analysis of a VAMP-seq dataset with multiple replicates, I had to:

• Create Expression plugins to drop the other replicate counts for each replicate
• Run the VAMP-seq plugin on each of these new single-replicate count data frames
• Merge the replicate scores
• Rename the score columns

This was a bit clumsy, so adding a field where count columns can be assigned to replicates in addition to the weights field would be a nice convenience.