In Provenance group, echodata_filename dim may duplicate the role of filenames dim

[emiliom]

I'm puzzled by the use of the Provenance dimension echodata_filename in addition to filenames. Note that echodata_filename is a more recent addition. I hadn't examined the Provenance attribute aggregation features before. As far as I can see, echodata_filename seems to perform the same function as filenames, so that it looks like an unnecessary duplication of the dimension.

I am not aware of a specific case where echodata_filename does serve a different function than filenames. But I've only inspected a couple of cases. A closer examination is needed.

For reference:

• filenames: A dimension and coordinate variable containing incremental integer indices for each source data file. Currently used only by source_filenames and, when relevant (AZFP only), meta_source_filenames. Assigned to every EchoData object during open_raw.
• source_filenames: a convention-based variable holding file paths to source data files. Assigned to every EchoData object during open_raw.
• echodata_filename: A dimension and coordinate variable added by combine_echodata, containing file names corresponding to the source data files used in the combination step. Note that it has no long_name attribute, so a longer description is not available.

Here's an example

This issue was motivated by PR #1116