Analysis classes

Represent each analysis with a separate class instead of a method in LooperMain. This finally closes shears#40 (closed). The three analyses have a substantial overlap in functionality (common event weights, filters, physics object builders). In principle, it can be factored out into a base class, so that the code is shared among all the analyses. But this would be a topic for a separate MR. Looping over events in the input dataset is delegated to a yet another class.

The way command line options are registered is adjusted accordingly. Different analyses now register different sets of options.

I checked that results of the main analysis, when run on ZZ→2ℓ2ν, are not affected. Haven't verified the other analyses.