[python] Add mechanism to register external Pyhton modules to dune-py
The function dune.common.registerExternalModule(module) has to be actively called in the downstream module to register an external package. This triggers the reconfiguration of dune-py in case dune-py has already been created.
Ideally registerExternalModule
is called before any code generation (and therefore the creation of dune-py takes place) but the implementation also works if a module is registered after. In that case, a reconfiguration of dune-py is triggered.
Fixes #274 (closed)