made eNearestNeighbour method default for 3D interpolation

In this merge request, the eNearestNeighbour method is made default for 3D interpolation (csv file to fld file), since this method is able to generate a perfectly 2D flow field in a geometry which is extruded in the third direction.

This change was made because we observed that other methods create tiny variations in the variables (u,v,w,rho) in the z-direction, so that they produce three-dimensional flow fields if we give them as initial conditions. However, the eNearestNeighbour method is able to generate a perfectly 2D flow field in a 3D domain (extruded in the z-direction).

It is important in cases that the geometry is extruded in the third direction and the initial condition must be two-dimensional in order to investigate the onset of three-dimensionalities in the flow field.