Added Levemberg-Marquardt and Steepest Descent
Added Levemberg-Marquardt and Steepest Descent

Both with documentation already in the inversion.solvers module.

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