Empty intervals do not properly compare

[AlexandreDecan]

Empty intervals do not properly compare because of the lower and upper bounds I.inf:

>>> I.empty() in I.open(-I.inf, I.inf)
True
>>> I.empty() in I.closed(0, 1)
False

Even worse, if AtomicInterval instances are directly created with an empty interval, containment depends on the values, not on the "emptiness" of the interval:

>>> i = I.AtomicInterval(False, 0, 0, False)
>>> i in I.closed(-3, -2)
False
>>> i in I.closed(-3, 3)
True

Similar issue holds when intervals are compared:

>>> i < I.singleton(0)
True
>>> i < I.singleton(-1)
False

Empty intervals should always be considered as included in other intervals, and should always compare to true when compared to other intervals.

[AlexandreDecan]

I'll work on this tomorrow.

• One should be allowed to create intervals with "reversed bounds" (ie. lower is greater than upper). Currently, this is prevented for no specific reason given that such intervals can be considered as a way to represent empty intervals.
• If we allow "reversed bounds", then we could represent empty intervals as (I.inf, -I.inf). This way, all operations involving an empty interval will be consistent (because everything is greater than -I.inf and lower than I.inf, meaning that empty intervals will be considered as contained in any other ones, as greater/lower than any other ones, and as having no overlap with any other ones).