Make extension modules work with PyNEST-NG and ensure NESTML support

[heplesser]

I have a first working version in my pynestng_module at https://github.com/heplesser/nest-extension-module/tree/pynestng_module

To see the changes required from extension modules working with current master and those for PyNEST-NG, it is best to compare with my modulemgr branch: https://github.com/heplesser/nest-extension-module/compare/modulemgr...heplesser:nest-extension-module:pynestng_module?expand=1

Changes mainly concern dictionary access.