Handle ``internals`` parameters in co-generation transformer

[clinssen]

To do: handle/collect internals parameters in the same way as parameters in the synapse/post-neuron transformer. See: https://github.com/nest/nestml/blob/eaf1b3cf8aa5170e6436c7a269047745640a621c/pynestml/transformers/synapse_post_neuron_transformer.py#L366