adds kwarg for ice density when calculating modelled geodetic mass balance

[pat-schmitt]

Here I add a new kwarg to the dynamic mu star calibration to provide geodetic_mb_ice_density. Previously it used an ice density of 900 kg m-3 when calculating the modelled geodetic mass balance out of the modelled volume change. But to actually match the volume change instead of the mass change we should use the same ice density conversion as was used when deriving the observed geodetic mass balance out of the observed volume change (e.g. 850 kg m-3 for Hugonnet).

Closes https://github.com/OGGM/oggm/issues/1474

• [ ] Tests added/passed
• [ ] Fully documented