Set collab solar fraction

[danielolsen]

Purpose

Simplify the getting of collaborative scaling next capacities. Once targets & resources are defined, collaborative_strategy_manager.data_frame_of_next_capacities() seems to be the top-level user-facing function, but there's no input for solar_fraction there. We could add an input here, pass that input to calculate_capacity_scaling(), and then pass that input to calculate_total_added_capacity(), or we could add one call to collaborative_strategy_manager.set_solar_fraction() before calling data_frame_of_next_capacities() and then we don't have to worry about propagating an input three levels deep.

What is the code doing

• We add a solar_fraction attribute to __init__(), defaulting to None.
• We add a new function _check_solar_capacity() to check inputs.
• We add a new method set_solar_fraction() which assigns values to the instance attribute.
• We modify calculate_total_added_capacity() to not take an input for solar_fraction, but to get it from the instance attribute instead.
• We re-use _check_solar_capacity() to replace functionality in TargetManager.__init__()

Validation

The code has been tested, and using the setter results in changes to the output capacities. The setter complains on bad inputs. All existing unit tests still pass.

Time to review

Five to fifteen minutes. This feature is not urgent, since I've already used the code for what I need it for right now, and I can always rebase and re-use it in the meantime. It would be nice to have in the codebase eventually though.

[dmuldrew]

looks good!