Raising interval to power zero should always give one

[andrewburbanks]

Raising an interval X to the integer power zero should give the interval [1, 1] provided that X is not empty, even if X contains zero. However, the current version delivers [0, 1] if X contains zero.

Example:

interval[0]**0
interval([0.0, 1.0])

[taschini]

You're right. I'll fix that as soon as I have a little time.