Implement plotdz

[wkearn]

This is a StreamObject method that plots the distance from the outlet on the x axis and the elevation of the stream on the y axis. We should be able to do most of this now, using streamquad_trapz to compute the upstream distance. The most challenging part seems to be splitting the stream network into segments, which we also have to do for #111.

This is used in the Getting Started guide section "Derive, modify and plot the stream network."