prototype function for calculating gzz by using kernelzz insert if sphere is None: continue in kernel functions
prototype function for calculating gzz by using kernelzz
insert if sphere is None: continue in kernel functions

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