improvement of interpolation in Bijker

When using the Bijker equation in a schematic test case (the flow over a Gaussian Hump), it appeared that the propagation of a bed form unexpectedly developed flat areas, which were not expected (See attached image, line labelled "lineair interpolation").

The issue was investigated, and it appeared to be due to the fact that in this equation, linear interpolation is used to calculate the transport rate as function of the bed shear stress (in the calculation of the Einstein integrals, subroutine integ.f). See the attached figure showing the calculated sediment transport as function of velocity.

Therefore, the routine was reprogrammed using spline interpolation instead of linear interpolation. Using this method, the propagation of the bed form behaves as expected.