Add compare concepts sets by source codes

[dimshitc]

Since mappings might be unequal or due to 1 to many mapping, target concept might drag redundant sources, it's critical to check the source codes the concept set is resolved into.
It would be helpfull if we can compare concept sets in this way.

[p-hoffmann]

Sounds very useful to me. I have a first implementation ready, but would like to see if there are other users interested in it, as Atlas 2.x was to my knowledge not providing it.