Reformulate URLs in metadata

[NathanCummings]

The url column in the metadata has the form s3://mast/level#/shots/#####.zarr, but users will most often be passing this to a call like:

shot_tree = xr.open_datatree(url, engine="zarr")

where url will want to be "https://s3.echo.stfc.ac.uk/mast/level#/shots/#####.zarr"

Should we change the urls to be in this form to make life easier, until we have resolvable persistent identifiers?