fix some issues when using python bindings with clang - some minor errors and a double registry issue

I added some comments about the reason for some of the changes needed to get the python bindings to work with clang. Whoever merges this might want to remove those again. The main issue is the use of static pybind11::class_ variables to avoid pybind11's double registry error. In the current tests it seems that the class variable is actually only needed the first time the class is registered so using already_registered is enough. If in a second module additional modules need to be added to the class this approach is not suitable and the type registry should be used. Finally, the same change possibly needs to be done in dune-localfunctions.