new zoom method in picamera

[carandraug]

231c5a7 added a new zoom method to PiCamera. This method is not part of the Camera interface and is also not used internally by PiCamera so not sure what the goal is here.

[iandobbie]

I implemented this as my pi2 was extremely slow (4-5s) to capture and image and download it. I thought the implementing this core zoom functionality to do ROIs might dramatically reduce this processing time as the system dealt with much less data, however it seems like it is just as slow so I think reverted back to doing the ROI myself as this was simpler and avoided any possible int-> float pixel position errors.

It is a tiny function and might be useful in future so I left it in.