Update SQL to use more robust method of fetching chromosome lengths

To paraphrase some discussion we had on this topic:

The original SQL is not a great way to get a list of chromosomes, for a couple of reasons.
1) The cs.name is not a controlled vocabulary; we have cases where they're called "chromosome_group" and the query won't pick them up.
2) We sometimes have chromosomes that we don't want to display; e.g. some species just have an MT chromosome, but it's not very useful to show that on its own.

The definitive way to check if something should be treated as a chromosome is not by name, then, but by attribute.