Improve visualization of StreamObject

[wkearn]

The current plotting method for StreamObject converts it to a binary GridObject and plots that. It works, but for larger DEMs, it gets very hard to see because every stream is a single pixel wide. The MATLAB STREAMobj.plot method splits the stream network into line segments and plots those segments. This is probably what we should do, but it does require a slightly complicated traversal of the stream network.

[wschwanghart]

Most functions to plot STREAMobjs use the function STREAMobj2XY. The function converts some data along the stream network into a nan-punctuated vector. Unfortunately, the name of the function is somewhat misleading and non-findable, but I found out only after I had written it that it is very useful for a number of tasks. That being said, I guess it might be useful to have such a function in Python as well.

Note also that there are other functions that plot STREAMobjs with variable line colors (see plotc or plotdz with the option 'color' set to some vector). I don't know whether matplotlib can plot lines with colors that change along the line (it seems it can). If not, then it might be good to have some similar approach as in splitbyattribute and num2rgb.