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
Lasagne using old version of seimsic.profile

The interactive VSP modeling Lasagne was using the old name of the
forward modeling function in seismic.profile
Merge branch 'master' of github.com:leouieda/fatiando
Merge branch 'master' into package-imports