Interpolation type for programs

[RomeshA]

Parameter scenarios now support setting the interpolation type. The same should be possible for programs - for budgets, coverage, and capacity. This should be supported for both scenarios and optimization. The likely implementation would be to add an interpolation type option to ProgramInstructions so that when the ProgramSet calls the interpolation, it can pass in the method if one is provided. This would then automatically work with optimization and bare scenarios (program scenarios using program instructions only). The Scenario objects (mainly used by the FE for program scenarios) could store the interpolation method directly for interfacing with the FE, and use it when constructing the program instructions. In practice, only one of spending, coverage, or capacity is likely to be set at a time in the program instructions, so specifying a single interpolation method applying to all of these quantities is probably simplest. Users needing to control the interpolation more precisely can always pre-interpolate their overwrites.

It's worth noting though, that this interpolation would be applied to the entire TimeSeries. It should therefore only be applied to quantities overwritten in the instructions i.e. it should not be possible for the program instructions to change how the historical spending is interpolated. That way, all Excel quantities will always be interpolated linearly.