Initial public release of analogueExtra

analogueExtra is a support package for analogue, providing functions that work with the main package but which are of lesser importance that fewer users will need.

The first release contains a single function plot3d.prcurve() for rgl-based, 3D plotting of principal curves. This functionality was originally in the main analogue package but I decided to remove it to allow easier installation of analogue on Mac OS X systems where some people have reported problems getting rgl installed and working.