Improved comments and API of the planetary gear constraint.
The ChShaftsBody constraint now can connect a more generic ChBodyFrame (ex. not only a ChBody, but also node of 3D FEM beams).
New constraint type: ChShaftsGearbox. It defines a transmission ratio between two 1D entities of  ChShaft type, and transmits the reaction of the gearbox to a 3D body that acts as the support truss.
Note that the more basic ChShaftsGear can do the same, except that it does not provide a way to transmit reaction to a truss body. This can be useful when simulating vehicles, where some parts of the driveline are gearbox connected to sprung masses, so one might want to capture the 'twist' effect when torque flows in the driveline.
Also note that this can also be seen as a ChShaftPlanetary where one has joined the carrier shaft to a fixed body via a ChShaftsBody constraint.
New constraint: ChShaftsGearboxAngled. It implements a gearbox with given transmission ratio between 1D input and 1D output, but with different directions in 3D space. Basically it defines a transmission ratio between two 1D entities of ChShaft type, and transmits the reaction of the gearbox to a 3D body that acts as the  support truss.
A typical example is the case of a gearbox with bevel gears, where input shaft and output shaft are at 90°.
Note that the more basic ChShaftsGear can do the same, except that it does not provide a way to transmit reaction to a 3D truss body.