r.out.gdal: add offset/scale options

[landam]

This PR adds two new parameters to r.out.gdal:

     offset   Assign a specified offset value to output bands
      scale   Assign a specified scale value to output bands

Example:

r.out.gdal input=random output=random.tif offset=100 scale=0.01

gdalinfo random.tif
...
Band 1 Block=100x40 Type=UInt16, ColorInterp=Palette
  Description = random
    Computed Min/Max=1.000,3661.000
  NoData Value=65535
  Offset: 100,   Scale:0.01
...

[metzm]

Explanation for my suggested changes:
Offset is only relevant if not zero. Scale is only relevant if not 1.
PR approval comes after your comments to my suggestions.

The motivation for my suggestions is to not clutter metadata with irrelevant information.

How about the need to set different offset and scale parameters for different bands in case a group is exported? I guess there is no such need, most probably you also don't see such a need.

[landam]

Offset is only relevant if not zero. Scale is only relevant if not 1. PR approval comes after your comments to my suggestions.
The motivation for my suggestions is to not clutter metadata with irrelevant information.

OK, make sense.

How about the need to set different offset and scale parameters for different bands in case a group is exported? I guess there is no such need, most probably you also don't see such a need.

You are right, it's limitation of the current implementation.

[metzm]

Looks good. Let's see if there is a need for different offset/scale values per band.