_subdiagrams only returns diagrams of the last dim in homology_dimensions

[nphilou]

The _subdiagrams method can (and currently should) take a list of homology dimensions list but will only return the diagrams of the last element in the list.

https://github.com/giotto-ai/giotto-learn/blob/cda44bd79760106185f3a2849d6b36dbfb0a5aae/giotto/diagrams/_utils.py#L17-L23

Should we remove the loop or return an array of [subdiagrams_i, subdiagrams_j] with homology_dimensions = [i, j] as param ?

Also, this method can be very useful for users so maybe should we make it public ?

[ulupo]

It is also worth noting that this impacts performance, however slightly. Perhaps removing the loop and taking an integer instead of iterable as argument is the cleanest option. @gtauzin what do you think? I also agree that the method could be made public, but in that case I'm not sure how we would make it visible in the docs page.

[ulupo]

Fixed by #439.