raster: Read raster for mask from env variable

[wenzeslaus]

Use environment variable GRASS_MASK to obtain the name of the raster map to use as raster map for auto-masking.

Mask can be in another mapset. If it does not exist, mask is not applied. The direct masking rules for values (zero and null is masked out) apply (as when using g.copy raster=xxx,MASK).

This is implemented as an alternative to using r.mask, i.e., r.mask does not get the name from the environment variable. Setting and unsetting is done by manipulating the environment variable. This is analogous to GRASS_REGION which cannot be manipulated by g.region and similar to g.copy used for MASK which cannot be mixed with r.mask -r.

Review code:

• [ ] raster/r.kappa/mask.c (mask to text including categories)
• [ ] raster/r.report/maskinfo.c (mask to text including categories)
• [ ] raster/r.random.cells/init.c (opens by name)
• [ ] raster/r.random.surface/init.c (opens by name)
• [ ] raster/r.surf.contour/main.c (opens by name)
• [ ] raster/r.volume/main.c (opens by name)

Use variable:

• [ ] testsuite/raster/raster_md5test.sh
• [ ] raster/r.mapcalc/testsuite/const_map_test.sh

Use r.mask.status tool:

• [ ] python/grass/grassdb/history.py
• [ ] gui/wxpython/core/gconsole.py

Update doc:

• [ ] raster/rasterintro.html
• [ ] vector/vectorintro.html
• [ ] raster3d/raster3dintro.html
• [ ] raster/r.mapcalc/r.mapcalc.html
• [ ] raster/r.mapcalc/r3.mapcalc.html
• [ ] raster/r.watershed/front/r.watershed.html
• [ ] doc/notebooks/parallelization_tutorial.ipynb
• [ ] doc/development/style_guide.md
• [ ] imagery/i.smap/i.smap.html
• [ ] imagery/i.pca/main.c
• [ ] raster/r.resamp.bspline/r.resamp.bspline.html
• [ ] raster/r.out.gdal/r.out.gdal.html