This adds support of GroundTruth protos in addition to
text files.
Fixes the ground truth generation code to skip outliers.
Improves the ground truth generation to use expected
relative poses instead of the solution.
Writes constraints including the roll and pitch from the
node for 2D SLAM, so that constraints correctly capture
the expected transform between the 3D poses of scan and
submap in 2D SLAM.
Related to #239.
Related to #310.
Wolfgang Hess