[MEDUSA] Code using character strings as switches cannot be put on GPU

[arporter]

The string options are a feature of the configuration interface of the external source code mocsy (Orr, J. C. and Epitalon, J.-M., 2015, "Improved routines to model the ocean carbonate system: mocsy 2.0", Geosci. Model Dev., 8, 485-499, doi:10.5194/gmd-8-485-2015) used by MEDUSA, and not part of the native MEDUSA code. It is therefore not practical to change the original source and we propose instead to implement a transformation to replace such switches with integers on a routine-by-routine basis.