Revert sphere.tf to old implementation

Using the kernel functions to calculate total-field anomalies was
resulting in much slower code (~3x). Reverting to this version won't
matter too much because this will be kept for testing purposes only when
the new Cython code is ready.
Dont use 0 in tensor.center_of_mass test

The sphere in the doctest has yc=0. This causes problems because the
result is sometimes 0 sometimes -0 depending on computer and OS.
Switched to yc=20 to avoid this problem.
Implemented gravmag.sphere in Cython

Slight speedup and uses less memory (doesn't create so many numpy
arrays). Total-field anomaly is calculated using the kernel functions.
Testing against numpy implementation in _sphere_numpy.py.