Implement all field components in fatiando.gravmag.sphere

[leouieda]

The sphere forward modeling module needs more fields. In 0.1 it only has the total field magnetic anomaly (sphere.tf) and the vertical component of gravity (sphere.gz). This module is used in the equivalent layer module for processing, so it is very important to have as many field components and derivatives here as possible.
The implementation is very simple (just copy what was done in the gz and tf functions) and the formula for spheres are simple to derive.

Fields needed:

• potential: gravitational potential
• gx, gy: the x and y components of the gravitational attraction
• gxx, gxy, gxz, gyy, gyz, gzz: gravity gradient tensor
• tfx, tfy, tfz: x, y, and z derivatives of the total field magnetic anomaly

All features implemented require a cookbook recipe to go along with it that demonstrates the functionality. To do this, simple create files in the cookbook directory that calculate the fields. You can just copy or extend this recipe https://github.com/leouieda/fatiando/blob/devel/cookbook/gravmag_grav_sphere.py