Allow for possibility to end with full NWP weight at end of blending

[RubenImhoff]

Currently, the STEPS blending procedure determines the weights for the extrapolation, NWP and noise components with generally an increasing weight for the noise component for longer lead times. Although this is a good approach with a stochastic representation of the uncertainty in the forecast, many users would like to end the blending procedure with an (ensemble) forecast that is identical to their (ensemble) NWP input. This allows for continuing with their NWP product for longer lead times instead of running the blending procedures for very long forecast horizons.

To make this possible, we will have to build in an option that, if set to True, linearly increases the NWP weights from the weight at e.g. 1 hour before the blending end to 1.0 at the end of the blending procedure (and the opposite, so decreasing weights, for the extrapolation and noise components).