More properties for CameraSpec (focal, fov,...)

[Conchylicultor]

CameraSpec/PinholeCamera could have additional properties like:

• spec.focal_px_wh: Returns the (fw, fh) focal length (in px)
• spec.focal_px: Returns the unique focal (when fw == fh). Implementation could be inspired from v3d.Transform.scale property: https://github.com/google-research/visu3d/pull/10/files#diff-6402e17c4cd486883434c2847d32baaac9b36f95fd76d3969852d7276645778d
• spec.fov: Returns the field of view

Because the fields can vary across each camera, maybe the fields should return a property to allow selecting sub-fields, like:

spec.focal_px  # FocalInfo[*s]
spec.focal_px.hw  # FloatArray[*s 2]
spec.focal_px.wh  # FloatArray[*s 2]
spec.focal_px.value  # FloatArray[*s]
# And also add option to remove the `*s` dimension ?
spec.focal_px.unique.wh  # FloatArray[2]

Not sure about the .unique, .value name though

[Achint08]

I would like to take up this task.

[Conchylicultor]

Thanks @Achint08 . Feel free to send us a PR!