Some MassBalanceModels rely on t_star if no y0 is provided

[pat-schmitt]

Some MassBalanceModels rely on the year of t_star if no y0 is provided (see here, here, here and here), but with the new glacier by glacier calibration to geodetic measurements t_star is set to None (e.g. here).
This could lead to 'hard-to-interpret' errors if someone tries out some of the tutorials, starting from prepro gdirs which use this glacier by glacier calibration.

So maybe we should think of a new default if no y0 is provided to the MassBalanceModels (e.g. y0 = gdir.rgi_date + 1).

[fmaussion]

I would be in favor of raising a meningful error and force users to set y0 explicitely

Explicit is better than implicit.