Check that an Interval is an interval

[alberto-paparella]

As suggested in the to-do here, I suggest to check that an object of type Interval is, indeed, and interval, not only for debugging, i.e., x<y (or, more generally, <(x,y) != 0; theoretically, this naturally generalizes to the many-valued case thanks to the third axiom of many-valued linear orders, i.e., <(x,x) = 0). Hence, I simply suggest removing comment on line 132 and deleting line 131.

[alberto-paparella]

Simply removing line 132 isn't enough, as it was an old constructor without type parameter T; instead, i updated the basic constructor, using a guard to improve readability. All tests pass and the code seem to follow the wanted behavior (notice how requesting that x>0 and x0). Anyhow, I request a review.

[alberto-paparella]

Please refer to #48 if you want to contribute.