add option to pass in a model.hh file to create.model()

Usage is as follows: model = create.model("elliptic", grid, 'ellipticmodel.hh', coefficients={u_b:exactB}) i.e. the path to a generated model file must be provided as a string (the name doesn't matter since the .so file name will be taken from the file itself). This is essentially for testing purposes so the model file can be modified directly without having to go through elliptic.py