autogenerate library model figures

[kmantel]

Figures for example models in models are saved as static files to be shown in the documentation at https://princetonuniversity.github.io/PsyNeuLink/Models.html. While underlying values are tested in test_documentation_models.py, the generated figures are not. If these figures were generated when building the docs, they would always be up to date, and package updates (currently only matplotlib is expected to affect them) could be accepted without manual verification.