Consolidate duplicated soiltemp() with air_to_soil_temp()

[fabern]

Following the DRY principle we should strive to de-duplicate code where possible.
This means, that (after merging #282) soiltemp() should be consolidated with air_to_soil_temp(), where the former is used in pmodel and the latter in BiomeE.

This could be taken care of, if we want to re-organize the pmodel side of the repo following the extensive refactoring done in #282