Class II alpha/beta chain pairing

[jfeala]

Great idea to make this a separate tool, I've found a need for MHC parsing outside of prediction as well.

For class II alleles with alpha/beta chains, NetMHC requires them to be entered as paired alleles, but HLA typing usually reports the chains separately. I have a simple function to contribute for this

[iskandr]

Hey @jfeala, sorry I didn't see this before. Do you want to add a helper which generates all pairs of alpha+beta chains? Sounds useful, want to open a PR?