added grib idx extract methods and test cases

[Anu-Ra-g]

The extract_datatree_chunk_index function is essential for mapping the idx and grib metadata to make a mapping from one to the other for a particular model horizon file. Also, the mapping method inside the workflow uses the extract_datatree_chunk_index internally.

[martindurant]

Sorry it has taken me so long to start looking at this.

Can you please provide a description for this PR, what the two main functions here are for, how they are expected to be used?

[Anu-Ra-g]

The extract_datatree_chunk_index function travel through each of the node in the datatree and extracts the grib metadata and returns it in the form of a Dataframe.

dt_instance = datatree.open_datatree(...)
extract_datatree_chunk_index(dt_instance, kerchunk_store)

[martindurant]

Yes, but why, what are you supposed to do with this new code?

No description provided.

[Anu-Ra-g]

The dataframe from this function and the dataframe from the parse_grib_idx function are used in the one-to-one mapping. This mapping provides a aggregrate view of the dataset.

[martindurant]

Please fill out the description of this PR at the top, saying why this new code should be included.

This mapping provides a aggregrate view of the dataset.

Please demonstrate how the code is supposed to be used.