Driver is copying data twice for each frame in Stream mode, not efficient

[MarkRivers]

When the data source is Stream the driver is currently copying the data for each image twice, which is inefficient. This is due to the class design:

• The eigerDetector class is derived (indirectly) from asynNDArrayDriver, so it has methods for creating and manipulating NDArrays.
• The streamApi class is independent of asynNDArrayDriver, so it cannot allocate or manipulate NDArrays.
• StreamApi.h defines a stream_frame structure. eigerDetector passes this structure to StreamApi when reading frames.
• StreamApi copies the data from the zmq buffer to the stream_frame structure.
• eigerDetector then copies the data from the stream_frame structure to the NDArray.

The driver needs to be restructured to avoid 2 copy operations, and the data should be copied directly from the zmq buffer to the NDArray.

[MarkRivers]

I have restructured the driver so that StreamApi.cpp and Stream2Api.cpp now create the NDArray directly, eliminating the second copy operation. This is in the pending #71 PR.