Ring Buffer: Added the 'at' method.

This method returns a pointer to the image specified by the given index.
uEye Camera: Integrated the new RingBuffer at() method.

This function is used in the start and stop methods.
Ring Buffer: implemented the map between buffer addresses and Images.

The 'getImageFromBuffer' method returns the address of an Image holding
the given image buffer.
Merge branch 'multiple_acquisitions' of https://github.com/RexusCWIS/camera_module into multiple_acquisitions

Conflicts:
	include/ring_buffer.hpp
Unit tests: Added tests for the 'RingBuffer' class.