Allow conversion from `VideoFrame` in 'gray16be' or 'gray16le' pixel format to ndarray

Created by: LHolten

Overview

Currently when trying to run code like frame.reformat(format='gray16').to_ndarray() an error is thrown ValueError: Conversion to numpy array with format 'gray16le' is not yet supported.

Desired Behavior

frame.reformat(format='gray16').to_ndarray() should return a numpy array with dtype numpy.uint16.

Example API

with av.open(path) as container:
    for frame in container.decode(video=0):
        yield frame.reformat(format='gray16').to_ndarray()

Additional context

In my current project I need to convert video frames to grayscale, but 'gray8' doesn't have enough depth.