Replace endogenous variables with patsy formulae

[mattwigway]

We should use patsy formulae rather than lambda functions to define endogenous variables. Lambda functions defined at the top level in Python fail after round-trip ser/deser with dill. Patsy formulae are just strings; if we used them for price-income transformations as well, we could remove the dependency on pickling altogether and serialize the entire model into an npz file.