ttools: add some more colour maps

Added MPL_Gnuplot and MPL_Gnuplot2 from
http://www.ncl.ucar.edu/Document/Graphics/color_table_gallery.shtml,
and one of the cubehelix maps, see
https://www.mrao.cam.ac.uk/~dag/CUBEHELIX/.
These were all imported from scavenged graphics using ttools.plot.LutSteal.
topcat: improve sky plot re-ranging

Refactored the auto-ranging logic so it's mostly in the AxisController
superclass rather than the subclasses.  Not sure why it was separated
before, hopefully there wasn't a good reason I'm missing.
Now the code is tidier plotting a new dataset on a sky plot will
re-range the axes as it does for a plane plot.
table: add N-d cuboid match engine

This is an N-dimensional Cartesian matcher which uses a cuboidal match
criterion, unlike most of the others which use ellipsoidsal ones.
Useful for associating points with pixels.  Requested by Kathy Vieira.