U/danielsf/cosmodc2/healpix

[danielsf]

This PR speeds up InstanceCatalog generation so that it is feasible to run on CosmoDC2. Specifically, speed-ups were achieved by

• applying a native_filter on healpixel ID
• only loading each cosmoDC2 quantity once, and holding all required values in memory
• using scipy.spatial.cKDTree to do the nearest neighbor search in SED assignment

With these optimization, a single LSST field of view (radius=2 degrees) was generated in ~ 70 minutes with a maximum memory footprint on Cori's login node of 20GB. This test did not use the sprinkler.

[danielsf]

I'll rerun the test to make sure I didn't accidentally break anything while responding to the review; I'll ping you when that passes.