CoppeliaSim Connector backend

There's currently no Connector backend for CoppeliaSim (formerly V-REP) in roboticstoolbox/backends/ — this has been a documented gap since the wiki's Backends page was first written. CoppeliaSim is a mature, cross-platform robot simulator with a free education licence.

Implementing this means writing a new Connector subclass against the API in roboticstoolbox/backends/Connector.py (see PyPlot, Swift, or ROS backends for the existing pattern).