[GRASS-dev] renamed "voxel" -> "volume" terminology

[Hamish]

re. r48435

"voxel" (volumetric pixel) is a standard and specific term which
should not be changed to the more generic "volume" in places where
it refers to individual, or collections of individual, 3D cells.

"volume" is more what's inside the faces of a 3D vector space, or
in describing 3D raster arrays in the general/abstract sense
(which many of the changes in r48435 seem to be, but I think it is
useful to introduce the voxel term there as it makes the
association clear, and there are limited chances to introduce the
term).


thanks,
Hamish