Also, the value for nuclear chromosomes is now `nucleus` instead of
`chromosome` for better alignment with the other value `mitochondrion`.

Note that I did not bother to make an Alembic migration for this, since
we don't have any installations besides my own yet anyway.

Genome assembly definitions for GRCh36, GRCh37, and GRCm38 are included
as JSON files in `extras/assemblies`.