Find a better way to get a reasonable precipitation gradient

[jlandmann]

Unlike the temperature gradient, which is straightforward to calculate from the meto grids input, the precipitation gradient needs a more careful treatment.
At the moment we use the grid as well to determine the gradient, however especially with the problem of spatial variability in precipitation messing up the gradient calculation.
We should think of a better solution, for example including only cells that intersect with the glacier outlines or that fall into the glacier basin.

For the glaciers where we have MB measurements, the preciptation gradient could be calibrated on extrapolated snow pit measurements.