geom.gp simple coordinate geometry

[Kevin Ryde <user42_kevin@yahoo.com.au>]

I've used gp for some simple coordinate geometry like area, centroid,
minimum area rectangle, and inertia (plane shapes, but a little 3D).
I had a go at collecting up some bits in a presentable form,

    http://user42.tuxfamily.org/pari-geom/index.html

Coordinates are any of vec, col, col matrix, complex, or complex quad,
since I've used all of these in different contexts :).  The main bits I
use are area of polygon, which is fairly simple, and inertia where I
found it easy to make one mistake after another so having checks for
final formulas is handy.