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. 
Yeah it won't work when table=snobs, but I'm not sure anyone is using this option. There's no way to (efficiently) pre-check the snobs table since the query for each candidate is based on RA, DEC.

Can you just put a check in `parse_args()` that checks if `opts.hasphot` and `opts.table=='snobs'` and raises an error if so?