matplotlib deprecated passing `norm` and `vmin/vmax` simultaneously.

> `discretize/View.py:1654:` MatplotlibDeprecationWarning: Passing parameters norm and vmin/vmax simultaneously is deprecated since 3.3 and will become an error two minor releases later. Please pass vmin/vmax directly to the norm when creating it.

This is potentially something we should take care for in `View()`.