Add Scenario

[nikhilwoodruff]

@anth-volk @PavelMakarchuk @MaxGhenis WDYT of this rough proposal.

Structural reforms have always been painful, and they are handled completely different than parametric reforms (and applying changes to the actual data is mostly impossible without modifying the simulation directly, e.g. increasing the earnings growth factor is impossible as a reform currently).

This Scenario class could supersede the existing reform class, make those things all possible, and at the same time add stability, type checking, validation, etc. Happy to write docs etc but want to make sure this has buy-in first.

[anth-volk]

Does this PR just create the empty structure? If so, I'm totally open to it, and I like the standalone class definition, though I wonder if "Scenario" is descriptive enough? (but then again, what is, right?). Otherwise, I'd be curious to learn more about how the modifier_function might work. It looks as if it will still be pretty strictly bound to the overall system, which I think is one of the bigger challenges that structurals present.