added spline interpolation

[Suvrat1629]

#228 added spline interpolation

[Suvrat1629]

@GrazianoCapelli updates?

[GrazianoCapelli]

Hello @Suvrat1629
first of all thanks for your pull request.
I apologize for the delay, yesterday I was very busy and I had no spare time to answer and to check your commit.

Now I checked it, and I found that the app doesn't run because the cubicSplineInterpolation method is not found.
Furthermore, in your code the function that returns the EGM correction has been renamed, thus the app cannot call the method anymore. As last consideration I cannot find the 3x3 matrix that should be used for interpolation.
Did you check it?

Unfortunately I cannot accept your commit as is.
Please run and test your code (and the app) before to pull it.