Permit separate .bim/.bed/.fam files

[carbocation]

It would be ideal to allow the end-user to specify separate .bim/.bed/.fam files, rather than requiring that they all be at the same path with the same prefix. There are plenty of instances where that assumption falls down. (For example, with UK Biobank datasets shared at an institution level, the .bim/.bed files will be separate from the .fam files.)