Re-write analytic steady state finder

[jessegrabowski]

In #1, I greatly simplified the steady-state solver. In doing so, I also removed the heuristic solvers that did "shallow" checks for analytic steady state solutions (e.g. r[ss] from the RBC Euler equation, or A[ss] = 1 from laws of motion of technology). These should be re-added as a part of a more robust system that can:

• Recursively search for a reduced form steady state in parameters only, with
• User-defined limits on depth (number of recursions) and compute budget (max time spent on sp.solve), and
• Write results into a GCN file steady_state block, so it doesn't need to be run multiple times.