Fix met workflow

- cleanup of download code
- fix dbfile.input.insert to work correctly

@mdietze @jam2767 @bcow 
met2cf.XYZ should take start_year and end_year, since there might be more years in the database than the user is interested in, or should take list of inputs_id's to get the files from the database.

met2model.XYZ should take start_year and end_year, since there might be more years in the database than the user is interested in, or should take list of inputs_id's to get the files from the database.
Rob, I appreciate you getting Josh's script to work, but as I've told Josh and Betsy numerous times the desired behavior isn't to have the download.* scripts be responsible for inserting their own records into the database (i.e. download.Ameriflux shouldn't need "con" as an argument). There needs to be a wrapper function, analogous to convert.inputs, that is responsible for doing the database insert so that every single download function doesn't need the same replicated code. Should we accept these changes in the short term, in order to get the workflow working, and then move this (now working) logic outside to the wrapper function on a future commit?