Update sphere.py - calculation of the gzz gravity gradient component
Update sphere.py - calculation of the gzz gravity gradient component

I modified the gz function for the purpose of calculate the gzz gravity gradient component.
rafaelmds/fatiando,eusoubrasileiro/fatiando_seismic,eusoubrasileiro/fatiando_seismic,fatiando/fatiando,mtb-za/fatiando,eusoubrasileiro/fatiando,drandykass/fatiando,eusoubrasileiro/fatiando,drandykass/fatiando,santis19/fatiando,cmeessen/fatiando,mtb-za/fatiando,fatiando/fatiando,santis19/fatiando,victortxa/fatiando,rafaelmds/fatiando,victortxa/fatiando,cmeessen/fatiando,eusoubrasileiro/fatiando