Record merged MH names

[standage]

Recent changes have de-emphasized support for looking up a database record using cross references: as MicroHapDB becomes the authoritative source for microhap data, the ability to connect to records in other databases is becoming less important.

However, in working on #102, I realized we may need to keep a record of merged microhaps. We're now explicitly requiring the MH names refer to the locus, and if a marker is published under a new name but overlaps with a previously published marker, we need to merge locus names while keeping a complete record of merged names.