Change focal length in sfmData file formats

[fabiencastan]

Description

Changes storage of focal in file formats:

• Focal is now in millimeter (instead of pixels).
• Focal is no more split on (x, y) but use focal and pixel ratio.

[simogasp]

it would be better to have the possibility to keep the focal length in pixel as it is the standard way in computer vision to define the intrinsics. without it will be hard to e.g. run evaluation benchmarks that probably won't have the camera information to recover the information in mm

[fabiencastan]

@simogasp The focal length is still stored in pixels. There is only a conversion to read/write to the file formats.