resample methods in to_model_ds

[tomvansteijn]

When modelling a large area it can be necessary to coarsen the grid beyond the default 100 m resolution dictated by the REGIS dataset. The method to_model_ds uses resample.ds_to_structured_grid, using nearest neighbour interpolation by default (the argument is not set).

I think it would be useful to have other scaling methods for the hydraulic conductances, such as the mean and harmonic mean. Have you considered to implement these in nlmod?