grass.script: Add support for named saved regions in RegionManager

[lindakarlovska]

This PR adds a new region_name parameter to RegionManager, enabling users to work with saved regions:

• If region_name refers to an existing saved region and no additional g.region parameters are provided, the saved region is used directly without modification.

  Example:

  >>> with gs.RegionManager(region_name="study_area"):
  ...     gs.parse_command("r.univar", map="elevation", format="json")
  

• If region_name is provided together with additional g.region parameters, the region is saved under that name and persists after the context exits, allowing for reuse in future operations.

  Example:

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

Existing usage without region_name remains the same as it was:
E.g.:

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

EDIT:
This PR introduces a new save() method that stores the current region under a persistent name, allowing it to be reused after the context exits. It also adds more docstring examples to clarify that saved regions were already supported through the standard g.region region= parameter.

[petrasovaa]

Wouldn't region= work?

>>> gs.run_command("g.region", n=228000)
>>> with gs.RegionManager(region="aaa"):
...     print(gs.read_command("g.region", flags="p"))
...
north:      228500
...

>>> print(gs.read_command("g.region", flags="p"))
...
north:      228000
...

[lindakarlovska]

Wouldn't region= work?

>>> gs.run_command("g.region", n=228000)
>>> with gs.RegionManager(region="aaa"):
...     print(gs.read_command("g.region", flags="p"))
...
north:      228500
...

>>> print(gs.read_command("g.region", flags="p"))
...
north:      228000
...

You are right @petrasovaa, this works normally. I intuitively expected an interface similar to MaskManager (so that there would be a region_name parameter, like mask_name), and then I didn’t see any example for region= in the docstring, so I didn’t realize that it’s already covered by **kwargs.

I think we don’t necessarily need to add a region_name parameter to the API then, adding an example to the docstring should be enough. At the same time, I added a save method, which makes it possible to save the contextual region for later use (it’s basically what I originally implemented via region_name, but now I moved it into a method, which I think makes more sense overall).