Adapt naming of transformation matrix

[mstoelzle]

The code on the transformation matrices is still a bit confusing. It would be probably better to put more emphasis on the notation than on the memory consumption:

https://github.com/tud-cor-sr/ros2-mocap_optitrack/blob/master/mocap_optitrack_w2b/src/WorldToBase.cpp#L136

This would be at least from a notation perspective correct:

T_0_P = T_0_M*T_M_P;

Please also add some more code comments to describe what these transformation matrices are exactly doing.