Added center function to mesher.volume
Added center function to mesher.volume

santis19/fatiando,mtb-za/fatiando,rafaelmds/fatiando,victortxa/fatiando,santis19/fatiando,mtb-za/fatiando,eusoubrasileiro/fatiando_seismic,eusoubrasileiro/fatiando,rafaelmds/fatiando,victortxa/fatiando,eusoubrasileiro/fatiando,drandykass/fatiando,cmeessen/fatiando,drandykass/fatiando,eusoubrasileiro/fatiando_seismic,cmeessen/fatiando,fatiando/fatiando,eusoubrasileiro/fatiando,fatiando/fatiando