Add option to set fixed focal length for point fitting calibration

[TomFarley]

Sorry if this is somehow already possible and I've always missed it, but it would be useful to be able to enter a fixed value for the lens focal length when performing a point fitting calibration.

Currently you can constrain Fx=Fy and enter the pixel pitch, but not enter a focal length value like you can for eg a virtual calibration. My fits often have a focal length value differing by over 1mm from the manufacturer's value (eg 25mm), which presumably is much larger than the manufacturer's error.

[ssilburn]

Not sure why I didn't reply to this at all....this is actually possible with newer versions of OpenCV (not sure when it arrived)..so this can now be added as an option! I should investigate the newer model available in OpenCV more in general.