Improve construction of Lagrangians

[jessegrabowski]

Currently, Lagrangians are required to be written as Bellman equations of the form:

U[] = u[] + beta * E[][U[1]];

The right-hand side can only include a discount factor and a single expectation, which must be of the left-hand side. Trying to include other variables, for example a stochastic discount factor Q[] = beta * E[][lambda[1]] / lambda[], will raise an exception. I forget why I did it this way, but this needs to be revisited.