DownslopeFlowpathLength: optional output in number of cells instead of unit of distance

[jfbourdon]

Adds a --cells optional switch to get the output as a number of cells instead of the default unit of distance.

I was looking in a way to get the opposite of that the D8 flow accumulation does, instead of counting for the upstream cells, counting from the downstream cells. This tool was already doing that, just not un the unit I wanted. Simply reversing all direction in a D8 pointer file and computing the flow accumulation cannot work directly because of the values at both ends of each flowpath (must be a 0 at the upstream and not at the downstream in this case).