Interval_t<T> maintenance and testing
- length() no longer requires the data type (T) to be implicitly convertible from an int.
- Add left() and right() methods for unchecked access to the endpoints.
- Add longest() for one-step creation of an interval of maximum extent.
- Add to_string (Interval_t).
- Add some unit tests. They are far from complete.
In a separate commit, one functional change:
- length () now returns NaN instead of 0 for intervals (-infinity, -infinity) and (infinity, infinity).