Stokes Cavity - non-newtonian (#748)

* Create stokesNonNewtonian.ipynb

Incorporating the exemplo for Non Newtonian fluid.

* Clean up non-Newtonian example

Remove unnecessary intermediate variables when constructing
non-Newtonian terms.

* Remove junk cells in non-Newtonian example

Remove junk cells in non-Newtonian example left over from previous
edits. Improve some of the narrative.

* spacing, spelling, and eps->0.25

* Fix notation

* Switch to MatplotlibStreamViewer

Co-authored-by: Daniel Wheeler <daniel.wheeler2@gmail.com>
Co-authored-by: Trevor Keller <trevor.keller@gmail.com>
Co-authored-by: Jonathan Guyer <guyer@nist.gov>