Greenland datasets: support for multiple geothermal flux datasets

[stephenprice]

Greenland datasets: Currently we only have the Shapiro and Ritzwoller geothermal flux dataset for thermal boundary conditions. At some point we'd like to test the sensitivity to other available datasets (which are summarized at the end of [this recent paper].(https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2019JF005252).

Links to the datasets are as follows:
Fox Maule
Shapiro and Ritzwoller
Rogozhina et al.
Greve et al.
Martos et al.