Fix documentation about TauDEM flow directions

[phargogh]

In our documentation, we sometimes say that we use TauDEM's flow direction conventions. Maybe this was the case once, but it is no longer the case. According to the TauDEM docs, they now use indicies 1-8 instead of 0-7.

We should update the docs, and maybe provide an example of how to convert a TauDEM flow direction raster to pygeoprocessing-style flow direction, or else maybe include a function (example) somewhere to convert TauDEM flow direction to PGP.

This came up in the NCI Nitrate Concentration work.