Add "hypergraph_union_find" and "mw_parity_factor" decoders to sinter

[Strilanc]

(This PR is just a merge-conflict-resolved version of #815)

This is a simple adaptation of the mwpf' python package to sinter. Just copied the _decoding_fusion.pyand modify it accordingly to let the decoder consider hyperedges. Tested using thegetting_started.ipynb` and they run decoding without a problem.

I haven't added any test cases for this, since I didn't find a test case for fusion_blossom decoder as well. Please let me know where should I put a basic test case for the new decoders!

The existing unit tests automatically iterate over the defined decoders and run them through some basic sanity checks.

Authored by @yuewuo