Minor verbose fix in GradientSolver
Minor verbose fix in GradientSolver

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