Modifications in joint getters by name

  - getJointByName is now const,
  - add a method getJointBy BodyName.
Fix the way weak pointers are stored in objects.

  - Joint::robot setter now takes a weak pointer as input.
  - robot_ is placed and thus initialized before body_ in Joint class.
Homogeneize coding style.

  - TypeWkPtr -> TypeWkPtr_t.