add notebook from tigr_parc as a tutorial in docs

[edickie]

there's some notebooks in tigr_parc that should probably actually go here..

notably.. https://github.com/TIGRLab/tigr_parc/blob/master/notebooks/06_playing_w_pscalars.py https://github.com/TIGRLab/tigr_parc/blob/master/notebooks/07_pscalar_plotting.py

-steps

1. convert notebook to executed markdown (with jupytext) so that docify can render it?
2. also add the orig python file users to download