added option to use gridded 2D topo data for geodetic_mb_calibration

[afisc]

Until now the function massbalance.mb_calibration_from_geodetic_mb() did use the height/elevation data of a glaciers flowline(s) to calibrate the massbalance parameters. This is now also possible by using gridded/distributed data via the additional boolean argument mb_2d, which is useful for the usage of distributed models like IGM.

• [ ] Tests added/passed (will still be added next week)
• [ ] Fully documented
• [ ] Entry in whats-new.rst