[MRG] Check if sample_domain have only unique domains indexes in check_*_domain

[apmellot]

In response to #16

@kachayev is that what you had in mind ?
Should check_X_y_domain and check_X_domain stay two separate functions?

A test should be added for this situation.

[kachayev]

Yes, Apolline, exactly!

Answering your question: check_X_y_domain and check_X_domain should stay separate as they are designed for different use cases/inputs. Their implementation, ideally, would re-use as much common functionality as possible, for sure, but that's, you know, in an ideal world :)

A few comments:

• i think we need new allow_ parameter so that users can decide when this should or should not be enforced
• it might be helpful to mention explicitly in the error message that the same domain should not be used both as source and as a target, otherwise it could be confusing

P.S. If I recall correctly, @rflamary had rather strong opinion around the indexing. I'm gonna let him to chime in here. Likely allow_ should be True by default ;)

[rflamary]

LGTM especially when keeping the default to True. This will allows us to set this parameter to DAestimators (by keeping the default to TRue) which can allow user to detect problems depending on their choice of numbering for domains in a future PR.