* Add an interface to parameterize the landmark constraint building

* Add a callback to receive April tag output, to transform it with
camera pose information and to store it into the SLAM object. The
covariance is rotated too, using CERES.

* Add a function to load an initial file containing the absolute pose
information of the tags that can be encountered. The file can have a
header and contain the tag ID, its 6D pose and the relative covariance
(43 elements) in this order.

* Add public argument to tag topic in launch file

* Add a new external_sensor group to gather sensor info

* Adapt documentation to new external sensor structure