Change SetSystem representation

[gmou3]

This is an attempt to represent SetSystem in a more informative and user-friendly way.

Previous:

sage: M = matroids.CompleteGraphic(7)
sage: M.bases()
Iterator over a system of subsets

Current:

sage: M = matroids.CompleteGraphic(7)
sage: M.bases()
SetSystem of 16807 sets over 21 elements

I think this is an improvement from resorting to calling, e.g., len, in order to to get some info, after you get struck with Iterator over a system of subsets.
Note that a SetSystem is not an iterator but simply an iterable (so __repr__ is also misleading). The previous developers were planning to make it into an actual iterator, as can be seen on L66 of set_system.pyx.

[github-actions]

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[tscrim]

This is an improvement given the current implementation. If it does become an actual iterator (or a "view" like Python_dict.keys()), it might know some additional information about its size, At that time, we can revisit the repr output.

[tscrim]

@gmou3 If this is ready for review, just let me know. I am ready to set a positive review if ready.

[gmou3]

Yeap, it is ready I think. Thanks for the review.

[tscrim]

Thanks.