Allow EGM96Reader to optionally initialize with WGS84 coefficients
Some propagation models are setup with WGS84 coefficients for ae and mu, rather than the standard EGM96 ones. The rest of the coefficients are the same, so simply overriding the coefficients at load time creates the appropriate gravity model.
(from redmine: issue id 209, created on 2015-06-09, closed on 2016-02-10)