Crossmatch returns weird results when RA and Dec are strings

[ctslater]

Ernesto Castillo found a really confusing behavior where some data he loaded from CSV was returning strange results when self-crossmatched. It turned out that the RA and Dec columns were being loaded as strings, rather than doubles, and for some reason the math in crossmatch managed to return non-zero results.

We should test for RA/Dec data types and raise an error, rather than letting these through.