Support for reductions on date ranges spanning across calendar years

[spencerahill]

Paraphrasing from @spencerkclark's #204 (comment); see there and previous in the thread for more.

Consider the edge case of taking the mean over days 360 to 380 of a simulation. The first six days would be in year one, while the last fifteen would be in year two. How might we support that? Sticking with 'ann' would not work, because aospy always groups within years to generate an annual time series, and then takes an unweighted mean across years (so because there are fewer days in year one than in year two, days in year one are weighted more heavily than they should be).

This would probably be best supported by a new time reduction pathway (to add to 'av', 'std', etc.). The operation would be to just take the time-weighted average over the times that fall in between the start and end dates specified in the main script. (We'd have to think about naming; the most natural would be 'av', but that's currently taken).

As of yet, this is not a known use-case, but it seems highly likely that it will eventually arise. We need to assess how difficult it would be to adapt the existing Calc.compute pipeline to support this.