Likelihood distribution aggregating over multiple years / decades

[lentinj]

From conversation with @vbartolino, @bthe - due to the sparseness of stomach sampling it would be useful to aggregate over more than a single year.

• Move done_aggregating_f from likelihood_data to g3s_time, so time has a say in when the stock is finished (and it's time to do a comparsion)
• A g3s_timegroup that can handle year intervals, like we do with age/length, and says we're done aggregating when final step of the end of an interval.
• Plumb that into likelihood_data, so things like year 1998:2000 are translated appropriately.

See also #29