Move DEP specific functions to a new package, perhaps DEPR

[jarad]

This package is primarily being developed using input-output files from the Daily Erosion Project (DEP). I recently found out that the DEP version of the WEPP executable is different from the provided version(s) via USDA. At the moment, I can't remember the specifics, but I think it has to do with limiting the output in the environment (env) file. Any functionality that is specific to DEP should be put in its own package rather than in this general purpose WEPP package.