Add population_genotype data to variation endpoint

This adds the population genotype frequencies and counts for variation. I've removed the duplicate lines from t/test-genome-DBs/homo_sapiens/variation/genotype_code.txt too.
This sub{} is unconventionally named, it could be readily confused with an object name. It's not essential but consider renaming it in keeping with the rest of the APIs, namely verb_lower_case or verb_ObjectType.