Add new `generate.functions.FractionalSum`

[Brewster Malevich]

Related to PR #31. Maybe needed to progress on Labor projections.

This issue is continuing a post by @jrising in #31 (comment):

While I have no problem with us having a Product calculation, I think there are still reasons to prefer a FractionalSum calculation object: you avoid the 1 - X problem, you can check that the fraction has no units and stays within bounds, the formatted code can be cleaner and the argument types more informative. But the partial derivative of a FractionalSum is a Sum of a FractionalSum and a Product, so you would still need to have this Product class anyway.

The goal here is to talk about whether it's needed, what's needed to make it work and what this thing might look like in the interface.

[Brewster Malevich]

Okay, @jrising, what are you thinking on how to handle the 1 - X problem, and bound checks with a FractionalSum.

What does the signature look like to you?

The yaml from our initial discussion:

timerate: day
climate: [ tasmax, tasmax_rcspline ]
models:
  - csvvs: social/parameters/labor/csvv_spline_jan2020_corrected/*.csvv
    specifications:
      minlost_hi:
        csvv-subset: 1 - 12
        functionalform: cubicspline
        prefix: tasmax_rcspline
        variable: tasmax
      minlost_lo:
        csvv-subset: 13 - 24
        functionalform: cubicspline
        prefix: tasmax_rcspline
        variable: tasmax
      riskshare:
        csvv-subset: 25 - 30
        ...
    calculation:
      - FractionProduct:
          - YearlyAverageDay:
              riskshare
          - YearlyAverageDay:
              minlost_hi
          - YearlyAverageDay:
              minlost_lo
      - Rebase

In our talk you also mentioned replicating something like Excels' SumProduct but we couldn't remember its name.

[James Rising]

Yes, that seems fine. Note that riskshare will need that csvv-subset-depenunit fix to specify that it's unitless. And you got the excel function name right.

[Brewster Malevich]

@jrising My impression was that csvv-subset-depenunit is something that should be implemented downstream in impact-calulations and not in open-estimate?

[James Rising]

@brews Totally. I just mentioned it because your config file isn't consistent without it.
Alternatively, we can offer people a way to use multiple CSVV files (which would naturally have different dependent variable units), associated with different submodels. But there are many ways that might work. Also for impact-calculations.