started eigenvalue and eigenvector computation for potential.tensor. Need to finish doctest
started eigenvalue and eigenvector computation for potential.tensor. Need to finish doctest

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