Maybe we should discuss whether land is np.nan or 0.0 in the internal model of v4?

Originally posted by @erikvansebille in #2387 (comment)

In the new XLinearInvdistLandTracer interpolator, land values are assumed to be np.nan, instead of 0, which is the current assumed value. I think this makes a lot of sense for many fields where no valid value exists on land and using 0 could result in ambiguous interpolation?