feat: a command for merging TR and SNP PGEN files

[aryarm]

...since PLINK makes this difficult to do

The benefit is that we could more easily compute LD between SNPs and TRs for @TaraMirmira and others

[aryarm]

It would be good to first hear from others in the lab about any roadblocks they encounter while trying to compute LD between SNPs and TRs. That will help us evaluate whether creating a command that merges TRs and SNPs is even the right way to go. We could have a meeting of some of the folks in the lab in March?