Spline interpolation of EGM Grid

[GrazianoCapelli]

Currently we calculate the EGM correction by picking 4 points of the EGM96 grid and using a bilinear interpolation.

GPSLogger/app/src/main/java/eu/basicairdata/graziano/gpslogger/EGM96.java

Line 186 in 0d358e2

// Bilinear Interpolation:

In order to have a better precision we should use the spline interpolation instead.

[Suvrat1629]

i have made an attempt at this issue and made a pull request, please review it.