make get methods const. Ticket #3273
make get methods const. Ticket #3273

keulYSMB/urdfdom,keulYSMB/urdfdom,robotology-dependencies/urdfdom,robotology-dependencies/urdfdom,shadow-robot/urdfdom,robotology-dependencies/urdfdom,shadow-robot/urdfdom,shadow-robot/urdfdom,robotology-dependencies/urdfdom,keulYSMB/urdfdom,keulYSMB/urdfdom,shadow-robot/urdfdom