add Guo spaces

...and a PColor based color map helper class for other pcolor stuff.
The parameterization is left open in the papers and generally left
to the user. It roughly matches the examples given.

An important lesson is that we need to support analysis from our color
representation, and need better support for spaces with a meaningful
center, like the Guo ones.
Merge pull request #2 from simonthum/master

add some Guo spaces