Identify bursts groups also using the burst number

Scheme proposed by @alexamici:

• remove the (slow) parsing of GCP on open_dataset(..., group=None) and only report available groups by burst_id, e.g. ["IW1/1", "IW1/2", ...]
• move the naming logic to an external function in xarray_sentinel that takes either the filename or the gcp dataset (whatever is more reasonable) and returns the mapping between the label and the burst_id

[piyushrpt]

Pretty much all groups use a ascending node based scheme since you don’t need to read a lot of metadata or do geometry based computations to do the labeling. Lot of them are in public literature, e.g - https://www.mdpi.com/2072-4292/12/18/2960/htm . Exact implementation doesnt matter as long as the labeling is consistent.

[alexamici]

@scottyhq & @piyushrpt current main is a big refactor and there is a defined way to access burst data via the burst index in the swath.

See: https://github.com/bopen/xarray-sentinel#crop-single-slc-burst-dataset

I plan to make a release soon, then we will try to add more ways to identify and access a burst.