Add new geothermal heatflux map and corrections

[talbrecht]

Add Geothermnal heatflux map from http://dx.doi.org/10.1029/2022GC010501 with depth levels and corrections from https://doi.org/10.1029/2020JF005598.

[talbrecht]

The dataset by Carina Häger et al., provides depth levels on a 10km grid. This can be nicely combined with PISM's bed thermal unit (e.g. -Lbz 2000 -Mbz 61) for a 2km layer divided in 60 equidistant sub levels.

[talbrecht]

The data are freely available from https://doi.org/10.5880/GFZ.1.3.2022.002 and at http://doi.org/10.22008/promice/data/topographiccorrectiongeothermalflux/v1