Normal gravity formulae and numerical values for reference ellipsoids 

This PR implements functions to calculate the normal gravity field (gravity of a reference ellipsoid). This includes the "centrifugal" component. 

The ellipsoid can be passed to the gravity functions as a keyword argument. Ellipsoids are represented by the `ReferenceEllipsoid`  class, which takes `a, f, GM, omega` as arguments. Other physical parameters of the ellipsoid are computed based on these 4 using properties. Instances of `RefereceEllipsoid` with the values for WGS84 are present in the module:

    >>> from fatiando.gravmag.normal_gravity import WGS84
    >>> WGS84.a
    6378137
    >>> WGS85.gamma_a  # normal gravity at the equator
    9.7803253359

Checklist:
- [ ] Make tests for new code
- [ ] Create/update docstrings
- [ ] Changelog entry
- [ ] Documentation builds properly
- [ ] All tests pass
- [ ] Can be merged