use precipitation area fraction to improve the precipitation sub-column distribution

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Include profiles of precipitation area fraction as new COSP input. With 
precipitation fraction profiles, the precipitation can be more appropriately 
distributed into the sub-columns.

The modified prec_scops.f90 is flexible in dealing with the precipitation 
overlap and fraction on subcolumns with or without modeled precipitation 
fraction. 

Original issue reported on code.google.com by yuyingzhang2011 on 8 Nov 2013 at 12:56