Merge pull request #135 from CasparJungbacker/dev-micro-cleanup

SB/KK microphysics cleanup

Split SB and KK schemes into separate modules, similar to how the different advection schemes are implemented. This has several benefits:
* The implementations of different sources are much more readable, as the subroutines are shorter.
 * Prevents using SB parameters in the KK scheme and vice-versa.
* More modular approach makes it easier to add new schemes.
 
Added more field_r's. When using FP32, all computations should now actually be done in FP32.
Some (opinionated) formatting.