Lsstdesc-gcr-catalogs

- add Run 1.2i object catalog (for general consumption) [thanks to wmwv]
- add baseDC2 v1.1 (for developers only) [thanks to evevkovacs]

- add truth catalog for run 1.2
- minor improvements to object catalog reader
- update the default catalog list

This tagged version will be the version that goes in to image simulation Run 2.0i.

This PR:
- renames coadd catalog to object catalog (but maintains backward compatibility)
- fixes bugs in the quantity info in the object catalog reader
- uses pre-generated column names to improve load time for object catalog for
- adds a new feature: composite catalog reader (still in experimenting stage)
- requires newer GCR, which adds the `len` method

We are happy to release GCRCatalogs v0.8.0.

Since v0.8.0-rc.2, we've added quantity info to the coadd catalogs and also and UM catalog (baseline catalog for cosmoDC2) reader.

This is the second Release Candidate of GCRCatalogs v0.8.0.

In this release we added cosmoDC2 v0.1 catalog and reader. Note that cosmoDC2 v0.1 is still under development so expect bugs/issues.