Fix pVACvector bug that would result in not all junctional epitopes getting tested on clipping

[susannasiebert]

When clipping, we would remove amino acids at the beginning and/or end of a peptide sequence but we wouldn't include additional amino acids to make up for the clipped amino acids. This resulted in not all potential junctional epitopes getting tested.

Example:

>peptide1					
GYIRQVGDFHQVIIRGRGHILPYDQPLRAFDMI     
>peptide2                           
SCEVGMAYSMEKYRRTDNMARVMVRYMKYLRQK

peptide1 -> peptide 2 junction for a 9mer junctional epitopes:

PLRAFDMI|SCEVGMAY

buggy junction when clipping the end of peptide1 by 1 amino acid (without this fix):

PLRAFDM|SCEVGMAY

correct junction (with this fix):

QPLRAFDM|SCEVGMAY