SE3 velocities

[g-ferrer-sk]

create SE(3) plus linear velocities by augmenting RBT.

[nosmokingsurfer]

Hi, Gonzalo @g-ferrer-sk

All SE3vel class changes are in the branch:
https://github.com/MobileRoboticsSkoltech/mrob/tree/feature/SE3vel

will add #xxx tag in next commit to automatically refer the issue