Add script to set genotypes for small deletions to missing

[ernfrid]

This is based on work from @cc2qe for the GTEx paper. This script sets deletion genotypes to missing if the library for the sample did not have sufficient power for deletions of that size.

[coveralls]

Coverage remained the same at 83.12% when pulling 3fc7275 on ernfrid:del_size_filter into eb0444b on hall-lab:develop.