Add min_peaks config option

[wsnoble]

Casanovo should have a configuration option that specifies the minimum number of peaks that are required to be in a spectrum before it will make a prediction. Currently, Casanovo will happily make a prediction (sometimes with a high score) even if the spectrum has only a handful of peaks. The default value for the analogous parameter in Tide is 20 and in Comet is 10. For de novo, I think we should use a default of 20. This is the minimum in the 9-species benchmark.

Note that, ideally, Casanovo would track and then report something like "Info: XXX spectra skipped because they have fewer than XX peaks."

[bittremieux]

Postponed until functionality from DepthCharge v0.3 has been integrated, which will make this very easy to implement.