Initial ReferenceEllipsoid class and WGS84
Somigliana and closed formula for gamma
Better docstrings and include module rst file
Changelog entry for normal_gravity module
Function to calculate Bouguer plate effect
Cookbook recipe for normal_gravity
Capital M
Uncomment file download
PEP8 fixes
Merge pull request #133 from fatiando/normal-gravity

Normal gravity formulae and numerical values for reference ellipsoids