HLA/MHC region should be skipped by default

[quattro]

FOCUS will perform association testing and fine-mapping at the HLA/MHC region by default. Due to the pervasive LD at this region it is troublesome both computationally and statistically.

FOCUS should operate similarly to FUSION and skip this region by default. I think adding a flag to explicitly include it, along with a warning/log message upon startup, would be useful.