Century model (from Cadisch/Rowe)

 This implemements the litter and soil organic matter submodels from the Century model (see http://www.nrel.colostate.edu/projects/century5/).

Development history:

• Century2 in Stella by Georg Cadisch;
• Re-implementation of the Stella model in Simile 2.91 by Ed Rowe
• Put on to a daily timestep by Ed Rowe;
• Converted to Simile version 4.0 by Robert Muetzelfeldt.