Adding cubehelix colormap

[Carnë Draug]

Hi

it was recently asked on the help mailing list about the cubehelix
colormap [1].  I have a cset that adds it to Octave and will push it
by the end of the week if no one opposes.

The main point against doing so is that it does not exist in Matlab and
and adding an extra one "just because", would open the door to everyone
wanting to add their favourite colormap (there are many, and people seem
to enjoy arguing about them way too much).  An alternative would be a
separate package for colormaps only.

Carnë

[1] 
http://octave.1599824.n4.nabble.com/Is-the-cubehelix-in-Octave-td4667819.html