numElements returns zero for empty Intervals

[bogovicj]

Changes the implementation of Intervals.numElements so that it returns zero "empty" Intervals
Where an Interval is "empty" if its max(d) < min(d) for any d

If we agree, then this definition should be documented.

see #300

[tpietzsch]

Looks good to me.
I would maybe change the a>0 : a ? 0 constructs to Math.max(a,0).

[bogovicj]

@tpietzsch

The latest push adds changes to the behavior of Intervals.union, fixing its behavior for empty intervals.
I called the methods with the old behavior unionUnsafe.

Alternatively, we could call the new methods unionSafe, and leave the old methods untouched and un-renamed.

[tischi]

@tpietzsch could this be merged now?