Circle#minmax does the wrong thing when center is a PointZero

[bfoz]

PointZero + @radius generates a Numeric when it should be generating a Point.

[bfoz]

The problem was that PointZero + Numeric resulted in another Numeric, instead of a Point, because there was no way to know what size to make the Point. Fixed by creating the PointIso class. Now adding a Numeric to a PointZero yields a PointIso with the same value as the Numeric.