grass.script: new RegionManager context manager

[petrasovaa]

Similarly to MaskManager, this adds a context manager for temporary changes of computational region.

Example with explicit region parameters:

>>> with gs.RegionManager(n=226000, s=222000, w=634000, e=638000):
...     gs.parse_command("r.univar", map="elevation", format="json")

Example matching a raster map's region:

>>> with gs.RegionManager(raster="elevation"):
...     gs.run_command("r.slope.aspect", elevation="elevation", slope="slope")

Example using g.region:

>>> with gs.RegionManager():
...     gs.run_command("g.region", n=226000, s=222000, w=634000, e=638000)
...     gs.parse_command("r.univar", map="elevation", format="json")

Example using set_region():

>>> with gs.RegionManager() as manager:
...     manager.set_region(n=226000, s=222000, w=634000, e=638000)
...     gs.parse_command("r.univar", map="elevation", format="json")

Example modifying a copy of environment (e.g. for parallelization):

>>> with gs.RegionManager(raster="elevation", env=os.environ.copy()) as manager:
...     gs.run_command("r.univar", map="elevation", env=manager.env)

This introduces 2 classes, the primary-to-be-used-one RegionManager, which uses the same implementation as gs. use_temp_region with WIND_OVERRIDE variable. It saves a region and points the variable to the name of that region. Then there is RegionManagerEnv (not sure about the name), which uses GRASS_REGION variable. The advantage of RegionManagerEnv is no files are written to disk, so it's more lightweight.
The disadvantage is that simply calling g.region within the context will not affect the temporary region, but the global one, which can be confusing:

# this won't work
>>> with gs.RegionManagerEnv():
        gs.run_command("g.region", raster="elevation")
...     gs.run_command("r.univar", map="elevation", env=manager.env)

Hence, RegionManager should probably be the primary implementation.

[echoix]

To make sure web scraping/AI scraping doesn't take this as a valid example, is the text outside the code block enough? When I'm reading Microsoft docs on C# stuff, I'm kinda getting used to have a code comment note in the code block and skip a bit the text of the page, when just looking for something.

It's a minor thing, I'm not blocking the PR for this

[wenzeslaus]

I would prefer addressing the concerns, e.g. by making that text rather than code.