Get correct constructor for numpy arrays

When points are parsed as a numpy array, the vertices returned by `compute_voronoi` are empty.

Specifically, using `obj.__class__` to detect type for numpy arrays will result in the empty arrays as their type `<type 'numpy.ndarray'>` is not used for initializing them directly. The `get_constructor` function added to `voroplusplus` checks specifically for a numpy array and returns the factory function, otherwise returns `type(obj)`.