specify a variant explicitly with chr_position

[benralexander]

I'm not sure if that's right; I would assume it was

chr11:1342454 would be interpreted as

var_11_1342454__/_, where the *s correspond to any alleles at that position

(note: there is some ambiguity here, since there might be multiple alleles at the same position. Resolve before beginning with Jason (ref also Josep) )