Add `streamplot` function

I just submitted a pull request to add a streamplot function to plot the streamlines of a vector field. This implementation is a modified version of the streamplot function posted to the list by Tom Flannagan [1].

This PR isn’t complete, but I wanted to submit it in order to get comments. In particular:

  • I haven’t added the pyplot function, only the axes method. It looks like the pyplot function should be generated by some boiler plate code.

  • I’m not sure what the docstring conventions are (the code isn’t terribly consistent), so I just (more-or-less) followed the numpy docs.

  • Some parameter that probably should be adjustable aren’t at the moment (e.g. max streamline length, some arrow properties).

  • I left the original copyright from Tom’s implementation, but I don’t know if the BSD license has anything to say about that.

I’m not totally familiar with the matplotlib architecture so I’m probably forgetting a number of things. Feedback appreciated.