Track NDArrayWidget.array changes for scene rendering

[benbovy]

ipygany.Component.array accepts ipydatawidgets' NDArrayWidget instances, which is nice if we want to send to the front-end some numpy arrays that may be then reused across multiple widgets (ipygany or other).

However, changing the value of the NDArrayWidget.array attribute has currently no effect on the rendered ipygany scene. It would be nice if this could be supported. This would be useful especially when NDArrayWidget.array is often updated (e.g., animated visualization of temporal data).