lensfun: Lens calibration data format

# Lens calibration data format

Possible embedded elements:

<distortion focal="number" model="name" k1="number" k2="number" k3="number" omega="number" a="number" b="number" c="number" />
  Declares the image distortion model for this lens. Distortion is specific for every focal distance, so you can declare multiple distortion entries with different focal values; the library will interpolate the parameters for intermediate focal lengths.
  See also:
    lfDistortionModel
<tca focal="number" model="name" kr="number" kb="number" />
  Declares the Transversal Chromatic Aberrations model for this lens.
  See also:
    lfTCAModel
<vignetting focal="number" aperture="number" distance="number" model="name" k1="number" k2="number" k3="number" />
  Declares the vignetting model for this lens at given focal distance, given aperture and given distance to subject. This kind of calibration requires relatively many sample points since it depends on three variables.
  See also:
    lfVignettingModel