Add Hardware-trigger mode to gentl/basler backends #77

In certain scenarios its usefull to have the camera(s) run in a hardware - triggered mode instead of freerun.
That way one can exactly control when the cameras record the frames and at the same time enables a synchronization between cameras: all cameras receiving a trigger signal record their frames at the same time.

I would suggest different modes of hardware-trigger:

1. fully external hardware-trigger - an external device will provide the TTL-like pulses to control the cameras. Simpler but requires external hardware.
2. use one of the cameras as clock: this camera runs in freerun mode and outputs its ExposureActive signal via GPIO, other camera(s) are set in triggered mode and use the clock signal of the main camera.