Added the permeability of free space to constants
Added the permeability of free space to constants

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