Option to not fetch candidates without photometry

I want to resolve #3 by adding a command line option to `get-des-lightcurves` that enables one to only fetch light curves for entries in `SNCAND` that have corresponding entries in the desired photometry table. This was designed specifically to address the large number of candidates without Sullivan photometry that Ravi was having to process while trying to query the `SNPHOTMS` table, but it is a general filter and can be used for other candidate tables, photometry tables, and schemas. 

I also removed the `NUMEPOCHS_ML` condition for the `SNCAND_LEGACY` table, since `NUMEPOCHS_ML` in the `SNCAND_LEGACY` table is computed WRT a different ML version (1) than the `NUMEPOCHS_ML` columns in the `Y2PROD` and `Y1RE(RE)PROC`  (2), which may cause confusion. 
@kbarbary Not sure if this will work in general. Can't `snobs` sometimes be used as the photometry table?