New var_expectation_model statement and re-design of var_expectation operator

Given a previously declared var_model, the var_expectation_model statement is
used to declare a way of forming expectations with this VAR (possibly using a
finite or infinite discounted sum). The var_expectation operator now takes a
single argument, the name of the var_expectation_model.

For the moment, this only works when the var_model is using equations
explicitly declared in the model block.