We need to write / refactor a DEM (or TauDEM) module that handles basic DEM analysis routines like pitfilling, flow directions, and upslope areas. This should mostly consists of small functions that wrap a call to TauDEM in the subprocess framework. This milestone is complete when:

1. The module exists and consists of modular functions
2. Each function has a comprehensive docstring
3. The code includes block-level comments
4. The module's outputs have been scientifically validated
5. The module is accompanied by a testing suite and passes all tests