d.vect: support for z height (geometry) colors added

After getting mad with Lidar points colorizing which till now
required a DB table with GRASSRGB attributes, I have
modified d.vect to support colors directly from z height (geometry).
Works for 3D points, lines (eg, 3D contours) and 3D polygons
(eg delaunay triangles):

# Spearfish:
g.region rast=elevation.10m
r.random elevation.10m n=5000 vector=random3d -d
d.mon x0
# display as black points
d.vect random3d
# display 3D points colorized according to z height
d.vect -z random3d zcol=gyr

# 3D contour lines
r.contour elevation.10m out=contour20m step=20
d.vect -z contour20m zcol=gyr

# generate 3D triangles
v.delaunay random3d out=random3d_del
# display 3D polygons colorized according to z height
d.vect -z random3d_del type=area zcol=gyr

Enjoy,
Markus