Restructure the data collection group tags to use the rsyncer table

For tomography multigrid the data collection group tags don't match between the fractions and metadata.
The fractions are in the form `base/Supervisor_SampleX` while the fractions are `base/visit/Supervisor/SampleX` so underscope vs slash causes issues.

This relates to a wider problem where the data collection group tag should be replaced with an rsyncer group to ensure the dcg registrations are consistent. This would make it clearer how fractions and metadata relate as it would avoid using file paths in the current way, but it requires some major code restructuring.

EPU sessions present some problems, in particular with the `Images-Disc2` behaviour where the metadata and fractions are related but need to become separate dcgs.

#791 needs fixing before this can be attempted

Resolve #791 first, then trace how data collection group tags, fractions, metadata, and the rsyncer table are represented. Pay particular attention to tomography multigrid and EPU's Images-Disc2 behavior; done means consistent DCG registrations without relying on the current file-path-based tags.

Written by the indexing model from the issue text.