Added safe_inverse to utils
Added safe_inverse to utils

Calculate the inverse of a matrix in different ways if it is sparse or
not.

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