Improve NeQuick performances

Both ITU and Galileo NeQuick ionospheric models rely on sampling electronic density profiles. Some other external models not implemented by Orekit do too (like ionospheric scintillation). This is done by calling repeatedly nequickModel.electronDensity() with the same date and effective ionization level and varying point position (latitude, longitude and height) resenting the sampling. This induces repeated calls to {Field}NeQuickParameters constructor which recomputes over and over again the Fourier time series for foF2 and M(3000)F2 layers, which adds a huge computation load.

In the ITU version, the Fourier time series should be computed once outside of the sampling loop and reused. In the Galileo version, the Fourier time series still need to be computed inside the loop as they depend on modip.