Update sphere.py - calculation of the gzz gravity gradient component

I modified the gz function for the purpose of calculate the gzz gravity gradient component.
Update sphere.py - corrected gzz function

I corrected a line indentation problem in the gzz function.
Merge leouieda master
Corrections on the title of interactive windows
Correction on the __doc__ of the function "trapezoidal"
Merge pull request #34 from birocoles/master

Corrections to docstrings of the modules "gui" and "gravmag.basin2d"