Add an option for a plugin to report return data after calling executePluginCommand. Also add python binding.

Currently the return data has to fit in shared memory, 8MB (Linux, Windows) or 1MB (Apple)
Preparation for streaming is added (to allow unlimited return data, see CMD_CUSTOM_COMMAND_STREAM_RETURN_DATA)

New C-API: b3GetStatusPluginCommandReturnData
PyBullet reports return data if available, in pybullet_executePluginCommand

For the plugin developer:
plugin can provide additional return data for executePluginCommand in the b3PluginContext, during executePluginCommand.
Lifetime of this m_returnData pointer is minimum of next call to the next executePluginCommand or plugin termination.

examples/SharedMemory/PhysicsClientSharedMemory.h

@@ -90,6 +90,8 @@ public:
 
 	virtual void getCachedMassMatrix(int dofCountCheck, double* massMatrix);
 
+	virtual bool getCachedReturnData(b3UserDataValue* returnData);
+
 	virtual void setTimeOut(double timeOutInSeconds);
 	virtual double getTimeOut() const;