Clean up ``NESTCodeGeneratorUtils.generate_code_for()``

[clinssen]

NESTCodeGeneratorUtils.generate_code_for() can potentially be refactored, as uuids in the module and model name are no longer needed after the introduction of module unloading in NEST 3.7.

However, the function also provides the option of generating code directly from a string in NESTML syntax, as well as returning the correct neuron and synapse model names in case of co-generation.

Removing the uuid feature would also mean dropping support for NEST Simulator versions prior to 3.7.