added new module and necessary refactorings

[Anu-Ra-g]

This PR adds a new module called _grib_idx.py with private functions and added necessary refactorings

[martindurant]

How does this one relate to #499 ?

[Anu-Ra-g]

@martindurant I've made the changes as you asked. But the pre-commit is failing on the unused imports when I made the function as user facing in kerchunk.grib2 module.

[martindurant]

the pre-commit is failing on the unused imports

You can fix this by including an __all__, which should be a list of strings to be considered as "public" in the module. Making this list has two effects:

• making the imports look like they are used
• defining what gets imported if you were to do from kerchunk.grib import *.

[Anu-Ra-g]

Should I add the original functions like scan_grib, grib_tree etc to the list or only the indexing functions? __all__ = ["build_idx_grib_mapping", "map_from_index"]

[martindurant]

Yes please, add all the public functions, just incase some one does do a "*" import.

[Anu-Ra-g]

@martindurant made the changes