Add useful built-in colormaps

[imagejan]

In addition to the "primary" color maps provided by ColorTables, it would be useful to also provide optimal color maps in the following categories:

• linear (with optimized local contrast), e.g. mpl-viridis or the linear_bgyw one from here,
• diverging (with both white or black center values),
• cyclic,
• isoluminant,
• protanopic-deuteranopic and tritanopic versions of the above.

See the Visualization wiki page and http://peterkovesi.com/projects/colourmaps/