Add a common base class for both HolonomicPlatformVelocity controllers
and add a property with which to choose one.
Make the acceleration limits also configurable via properties.

* The old controllers are called SynchronousNJointController
* Added the common base class NJointControllerBase

* AsynchronousNJointController provides rtRunIterationBegin and rtRunIterationEnd.
 The former is called before and the second after all SynchronousNJointController are executed

Apply changes from 0e87721a ("made velocity mode for kinematicunit configurable")

Move loading the robot from RobotUnitModule::RobotData::_preOnInitRobotUnit to RobotUnitModule::RobotData::_initVirtualRobot and call this function in RobotUnitModule::ModuleBase::onInitRobotUnit

Change other components to load the robot earlier

Refactored the whole RobotUnit (split it in modules, cleaned up code, added doxygen, removed undefined behavior, reduced data sharing)