Add Camera.isOpen() method

From time to time I have the problem, that I want to make sure whether or not a camera I am using right now is opened/accessible, without trying to grab a picture. This might make sense for example before we try to record a number of triggered images (splib) or a HDR exposure sequence.

As of right now the easiest way I see is to try and grab an image from the camera and if that fails try to open the camera. I think most camera manufacturers implement a function like isOpen in their camera API that could be checked instead. But since these again might not be the same for all manufacturers I suggest we add a method to our template that gives a clean way to check this...

I will create a small merge request that adds this functionality to the camera template and the official basler wrapper. I hope it should be done in less than 5 lines of code for every camera implementation we have...