Accept subgroup-based matches.h5 files

[jatentaki]

To my best knowledge, you currently only accept matches.h5 files with keys in form of {filename1}-{filename2}. I would request that support for {filename1}/{filename2} is added, which is interpreted by HDF as subgroups and has the advantage that hashing is performed in stages (first filename1 then filename2). This has no impact on IMW submissions which only have thousands of file pairs, but single-stage hashing starts choking at > ~300k keys. It would be a quality of life improvement, because I could release my code with a simple script which is both compatible with IMW submissions and scales to large scenes (currently it's mutually exclusive).

[ducha-aiki]

Could you do PR for that, please? :)

[etrulls]

I asked Michal to make an issue so we don't forget about it, but let's wait until we know if there are significant changes for the 2021 version.