Support NumPy datetime64 for time scales

**Problem Description**

Currently times are expected to be expressed as floating point seconds since the epoch. Since the scale system was written for Chaco, NumPy has grown datetime and timedelta dtypes which are whole numbers of date or time units (from attoseconds up to years).

It should be feasible to use these directly as values in scale systems rather than having to convert to floats. This might possibly require some generalizing of the `Range` class as well.