Given a colourmap object m, which maps numerical values to
colours, this function interpolates the mapping,
yielding a new colour map.
This makes it easy to build a colour map that
has smooth gradation between different colours or shades.
First specify a small vector of numbers x which should be mapped to
specific colours y. Use m <- colourmap(y, inputs=x)
to create a colourmap that represents this simple
mapping. Then apply interp.colourmap(m)
to obtain a smooth transition between these points.