Add interface to flip endogenous and exogenous variables.

Calls to change_type can be done in a file named model-inversion-setup.inc in
the current folder. The file is included, between the declaration of the
exogenous variables and the model block, in the aggregate mod file if and only
if the macrovariable InvertModel exists and is equal to True. To invert a model,
the user just has to run the aggregated mod file with -DInvertModel=True on the
command line.