r.colors: add offset and scale

[metzm]

adding offset and scale enables r.colors to add e.g. the celsius rules to a raster map with units = Kelvin * 50

[neteler]

Works nicely (I only tested the scale parameter):

# North Carolina sample dataset
g.region raster=lsat7_2002_40
RED=lsat7_2002_30
NIR=lsat7_2002_40

# scale NDVI with 100 and store as integer (CELL)
r.mapcalc "lsat7_2002_ndvi_int = round((float($NIR - $RED) / ($NIR + $RED) * 1.0) * 100)"

r.info -r lsat7_2002_ndvi_int
# apply NDVI color table, scaled:
r.colors lsat7_2002_ndvi_int color=ndvi scale=100

d.mon wx0
d.rast lsat7_2002_ndvi_int