Add NEST code generator helper as workaround for module reloading

[clinssen]

This adds a helper function that generates modules and modules with unique IDs, bypassing the issue that NEST cannot yet do module unloading/reloading.

This will be used in interactive environments: from Jupyter notebooks and NEST Desktop.

Todo

• Move function to NESTCodeGeneratorUtils class (as static method)
• Use os.tmpfile to create temporary NESTML model files.
• Adapt all Python notebook tutorials to use this function.

[pnbabu]

Looks good to me!

[clinssen]

Thanks for the review!