Resolve "Store calculated errors"

The errors of the extrinsic calibration are stored in the following way:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE PETRACK>
<PETRACK VERSION="0.8">
    ...
    <EXTR_CALIBRATION>
        <REPROJECTION_ERROR MAX_DH="1.4902721643447876" MAX_PH="1.4046432971954346" MAX_PX="7.7531633377075195" SD_DH="0.42475560307502747" SD_PH="0.34359094500541687" SD_PX="2.1862344741821289" SUM_DH="0.89451408386230469" SUM_PH="0.98140132427215576" SUM_PX="4.6230463981628418" USED_HEIGHT="180"/>
    </EXTR_CALIBRATION>
    ...
</PETRACK>

• Adapt version (since .pet-file changed)

Closes #108 (closed)