Use ModelRaw for python API

[carrascomj]

There is not a very good reason to use Model instead of ModelRaw for the python bindings (at pyo.rs).

• Drawback: getParameter would be O(n) (or better, whatever find is) instead of O(1), but that can be solved easily, if needed (not really, the usual number of parameters in SBML documents do not justify the use of a more complex data structure in this case).
• Advantage: using ModelRaw would allow for adding writing capabilities to the python API.
• Advantage: less String allocations.