Added todo note
gravmag.tesseroid now faster than C Tesseroids thanks to replacing list building in _need_divide with numpy fancy indexing and conditionals
Added module for gravity effect of half a spherical shell
Added unit test for tesseroid potential. failing
Added half shell module import to gravmag init
Corrected bug in gravmag.tensor.center_of_mass when using 1 window
Fixed failing test for tesseroid. Was using too large shell model