Add two-stage build process to PyCont

I just modified the ContClass::loadAutoMod method to enable a two-stage build process.
This is similar to what is done in the Radau integrator code and this allow using custom C code in the rhs specification.

The main difference with the integrator two-stage process is that once the library has been compiledis also need to be imported using the "importAutoLib".
I could also add this method into "makeAutoLib" which currently combine the source creation and the compilation.