Improve automatic model documentation generation

[clinssen]

Improve model characterization on https://nestml.readthedocs.io/en/latest/models_library/index.html, especially the plots.

Add pulse current stimulus plot.

Refactor script to be in a separate directory (and equally applicable to NESTML models as NEST built-in models).

To do:

• [ ] automatic rheobase detection

[jessica-mitchell]

@clinssen can you also link to the script that generates the plots? thanks