added initialization of snags and some new resource unit variables.
Example: model.initialization node:
<snags>
<swdC>12000</swdC>
<swdCN>120</swdCN>
<swdCount>50</swdCount>
<otherC>4000</otherC>
<otherCN>30</otherCN>
<swdDecompRate>0.05</swdDecompRate>
<woodDecompRate>0.05</woodDecompRate>
<swdHalfLife>30</swdHalfLife>
</snags>