Ilrmsd onechain clarified

[mgiulini]

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Closes #920 by retrieving the chains in ilrmsdmatrix and stopping the execution if no contacts have been retrieved.

this changes also the caprieval module, in which now the function check_chains is externalised in the libalign library. Additionally, this closes #973 by setting ligand_chains to an empty list by default.

[rvhonorato]

lgtm

[rvhonorato]

Added a few #TODO markers so w ecan handle it before the release and you don't have to worry about it now 🙂

The mocked previous_io needs to use self.path to avoid contamination - see 9226d14, but good as is - we can make it proper later.

[VGPReys]

🚀