tensor.center_of_mass no longer returns sigma

sigma was a (bad) measure of uncertainty and wasn't actually used
anywhere other than internally by the function.

Also improved the doctest by using a sphere model (easier to know the
true center of mass)
tensor.center_of_mass no longer returns sigma

sigma was a (bad) measure of uncertainty and wasn't actually used
anywhere other than internally by the function.

Also improved the doctest by using a sphere model (easier to know the
true center of mass)
Merge branch 'tensor-center-of-mass' of github.com:leouieda/fatiando
into tensor-center-of-mass
Merge pull request #81 from leouieda/tensor-center-of-mass

tensor.center_of_mass no longer returns sigma