#19 Move aspect-ratio and cropfactor into <calibration> tag

[seebk]

Proposal from the mailing list:
https://sourceforge.net/p/lensfun/mailman/message/33186260/

We could move the aspect ratio and crop factor into the calibration tag and allow several calibration sections per lens. This could solve the problems of multiple lens entries for different crop factors or aspect ratios and even for RAW/JPEG.

For example:

<lens>
    ...
    <calibration aspect-ratio="4:3" cropfactor="1.536" RAW="true">
        <distortion ...>
        ...
    </calibration>
    <calibration aspect-ratio="4:3" cropfactor="1.0" RAW="true">
        <distortion ...>
        ...
    </calibration>
</lens>