Use a static packet buffer in DMXConnection

[rec]

Hello, and thanks for this very useful library!

This pull request should be completely backward compatible.

If you construct DMXConnection with the new use_numpy parameter set to True, then numpy.array is used to construct the buffers instead of array.array.

There are many advantages in general to numpy.array and two ones specific to this project, using the fact that taking a slice of a numpy.array creates a reference, not a copy, so modifying the slice modifies the original.

In the existing array code, a new buffer is created on each DMX frame update and then discarded.

In the new numpy codepath, a fixed np.array buffer called _packet is constructed with _packet_begin and PACKET_END already written into it, and then dmx_frame is just a slice out of that.

So when you manipulate dmx_frame, you are directly writing into the final packet which is sent to the controller. No copies or allocations are performed.

The second advantage is that you can now construct "lighting instruments" by taking out little slices from DMXConnection.dmx_frame, which is what I'm doing in my code!

Both of these take advantage of the fact that taking a slice of a numpy.array creates a reference, not a copy.

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I tested all four cases, with and without numpy installed, and with and withoutuse_numpy=True.

I both ran the nose tests, and lighted a lamp on my table. :-D