Automated dem tests

Currently, the CI/CD pipeline does not include the test for the "dem" module. This is because of the difficulty of getting TauDEM installed within the pipeline. However, this module should eventually have automated tests.

If "dem" migrates to pysheds, that should resolve this issue. But if TauDEM is retained, then the pipeline should be updated accordingly. One possible option for TauDEM tests could be to build a docker image (possibly externally) that includes a TauDEM install.