Haxby color map

Create a colormap with Haxby's colors, commonly used for bathymetry and marine gravity data.

Colormap is based on the colors used by W. F. Haxby's Gravity field of World's oceans, 1985, developed for geoid and gravity maps. The version used here is formed from a linear interpolation of the GMT color table used by MB-System by David W. Caress and Dale N. Chayes (http://www.ldeo.columbia.edu/res/pi/MB-System).