place variants in center of acceptable range when possible, but check for other options

[zachary-foster]

Currently the code tries to place a variant in a single location in the crRNA, checks if that one loction could work. It decides if the set of diagnostic variants will work or not based on only one possible location of the variant in the crRNA. IT should testall possible locations for the variants within an acceptable range, starting with the middle position and returning the first valid one it finds. Probably another loop in find_diag_region.