Ionospheric delay and variance for GPS signal

Function for the calculation of ionospheric delay and variance, application to GPS signal

This function performs the calculation of ionospheric correction for GPS signal, in terms of:

- ionospheric delay, on the basis of Klobuchar model, i.a.w. IS-GPS-200,

- ionospheric variance, on the basis of ionospheric delay, obliquity factor and geomagnetic latitude, i.a.w. DO-229.