F cleanup indigo

It seems that in every C++ function of sr_hand that needs a string we are passing a copy of the string. That is unnecessarily innefficient. The C++ idiomatic/efficient function prototype would be :
void SRSubscriber::cmd_callback( const std_msgs::Float64ConstPtr& msg, const std::string &joint_name )
We should update that when we have time. Not necessarily in this pull request.