Add an "automated fix" feature

[tfrancart]

This is a proposal to add automated fix to Issues in qSKOS.
Please see this commit tfrancart@52071b5.

Each issue is enhanced with a "getFix" method that returns an "IssueFix" capable of computing the suggested sets of triples to add and triples to delete in order to fix the issue.
Basically the idea is to implement something similar to https://github.com/NatLibFi/Skosify/wiki/Validation.

This is illustrated for the moment only for the ValuelessAssociativeRelation; see an example in the test class "ValuelessAssociativeRelationsFixTest".

I would like to get your feedback on the overall architecture before going forward and implementing it for other issues, and proposing this into a PR. What do you think of this ? would you be interested to merge this in qSKOS ? I prefer to wait for you green light before going further.

Cheers