Fix array validator

In my eyes, the behaviour of the Array-trait's validator is inconsistent with the documentation: The documentation claims that it will coerce values as if given to the `np.array` function. However, in fact, it rejects everything that is not either an `np.ndarray` instance itself or a Sequence. `np.array` on the other hand is far more lenient.

This is particularly striking with scalars: Python scalars would get converted to zero-dimensional arrays which fulfil most array-requirements perfectly fine!