Use a proper name for the step param in steepest
Use a proper name for the step param in steepest

Was calling it  stepsize, which is wrong because it isn't. Use the name
"beta" because it's used in the docstring equations.

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