Exact two layer solution

[hdrake]

I tried to implement the exact solution to the two-layer problem (sum of a fast-timescale exponential and a slow-timescale exponential) rather than the one-exponential deep-layer model we have now.

For some reason, introducing a second timescale makes the problem unsolvable by Ipopt via JuMP. The algorithm solves successfully for time horizons less than about 2150, but then I get errors of "search_direction_too_small" or "iterations_diverging". I'm going to try out NLopt and see if it's any better.