Century model (weekly) (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;
• Converted to Simile version 4.0 by Robert Muetzelfeldt.
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