Refactor distrib funcs and add them to OGGM

[fmaussion]

This moves @anoukvlug 's functions out of the notebook and adds them to the OGGM sandbox.

The algorithm did not change but it is now considerably faster (moved from xarray to numpy and use of np.digitize)

Excellent algorithm @anoukvlug , very clever

[fmaussion]

animation.mp4

[fmaussion]

TODOs: dealing with various output files (past + projections) and with RGI date (probably some small refactoring involved)

[fmaussion]

OK so in the process I uncovered a bug in distribute_thickness_per_altitude -> the results changed slightly towards thicker ice thicknesses in the center and less thick at the border. The bug was that we were also estimating non-zero thickness outside of glaciers and as the result the smoothing would increase the thickness at the border.