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. 

Functions:
* `gamma_somigliana` calculates using the Somigliana formula
* `gamma_somigliana_free_air` calculates at a height using the Somigliana formula plus the free-air correction
* `gamma_closed_form` calculates anywhere using the closed form expression.
* `bouguer_plate` calculates the gravitational attraction of the infinite Bouguer plate

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:
- [x] Make tests for new code
- [x] Create/update docstrings
- [x] Changelog entry
- [x] Documentation builds properly
- [x] All tests pass
- [x] Can be merged