ENH: refactor and cleaned up kmer_hash.hpp - combined hash and hash_prefix.
ENH: refactor and cleaned up kmer_hash.hpp - combined hash and hash_prefix.

ParBLiSS/kmerind,ParBLiSS/kmerind,ParBLiSS/kmerind,ParBLiSS/kmerind,ParBLiSS/kmerind,ParBLiSS/kmerind