Open constraint getters

[fabienrohrer]

In order to create advanced motor control such as PID, it is require to get precise information about the motor positions. The shortest way to get this is to use the dedicated Bullet API (currently missing in ammojs).

I propose here to open few very useful getters to do so.

To illustrate this requirement, please check the C++ Gazebo example here: https://git.lcsr.jhu.edu/dscl/ascent_mk2_hri2016/-/blob/master/src/gazebo/gazebo/physics/bullet/BulletHingeJoint.cc#L218

I tested successfully locally the compiled binaries in my application.