Remove unnecessary intermediate class ChLinkGeometric.
Collect all definitions of link type identifiers into the base class header file ChLinkBase.h
First draft of revolute-spherical composite joint.
Add more functions for the revolute-spherical joint:

- accessor methods
- implement functions for caching multipliers for warm starting
- cleanup
Add a demo to illustrate the use of a revolute-spherical joint
Add functions to obtain the Y and Z axes of the coordinate system corresponding to a (normalized) quaternion.
Provide an alternative initialization function for the revolute-spherical joint.

Add more comprehensive comments.
Merge branch 'feature/rev_sph' into develop
Add wrapper class ChLinkLockUniversal
Add several ChCoordsys transformation methods.

These functions parallel those for ChFrame and provide support for transforming vectors (points or directions) and coordinate systems to and from the parent coordinate system.
Fix bug in initialization of the revolute-spherical joint.
Immediately cache the frame passed to ChBodyAuxRef::SetFrame_REF_to_abs()

Without this, calling GetFrame_REF_to_abs() before a call to Update() occurs, returns the identity transform.