BUG: Remove equality comparisons with None.
ENH: Add new ArrayOrNone and ArrayNumberOrNone traits.

These avoid doing implicit comparisons of arrays with `None`.
BUG: Avoid a comparison of arrays with None.
ENH: Use the new ArrayOrNone and ArrayNumberOrNone traits.
Merge pull request #146 from enthought/fix/is-none

Avoid ndarray comparisons with None.