Kinematics wishlist: robust/global IK, symbolic IK, DH model porting, xacro error reporting

[petercorke]

Cluster of longstanding kinematics ideas, migrated from the wiki's Contributing page (which had drifted out of sync with actual status — see the wiki cleanup discussion). Not scoped/prioritized; pick any item and open a PR, or comment here to discuss approach first.

• Robust/global-search IK (current numeric solvers are local — IK_NR/IK_LM/IK_GN/IK_QP in roboticstoolbox/robot/IK.py)
• Symbolic IK solutions — interface to OpenRAVE's IK generation toolset, and/or integrate with SYMORO (Python 2 only currently)
• Improve the URDF/xacro subsystem to report parse errors with line numbers
• Port additional DH-convention robot models from the MATLAB toolbox that haven't been carried across
• DHFactor (symbolic factorization of a DH-parameter chain)