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.
Let API determine toplevel coord sys

Don't assume it is called 'chromosome' as other names are valid.
Let API determine toplevel coord sys 

Don't assume it is called 'chromosome' as other names are valid.
Merge pull request #76 from nicklangridge/bugfix/chromlengths

Update SQL to use more robust method of fetching chromosome lengths