Add two more query methods to SDSS

This PR adds `query_specobj` and `query_photoobj`, methods for querying the SpecObjAll (spectroscopic data) and PhotoObjAll (imaging data) tables respectively.

`query_specobj` makes a query based on at least one of plate, mjd and fiberID, and returns the results in a table.

`query_photoobj` makes a query based on at least one of run, camcol and field, and returns the results in a table.

The PR also includes tests for the new methods.