Curve plotting

[chrishavlin]

This PR adds a new Curve object so the user can plot arbitrary curves along with other rendering. Here's the output from the new example that plots a circle in the x-y plane:

[chrishavlin]

still a few things to do:

• alpha control doesn't seem to work
• I want to be able to add collections of lines

[chrishavlin]

latest commits added a curve collection object to add multiple curves at once that are will all have the same width and color. I also changed the new example, amr_volume_rendering_with_curves.py, to calculate and add streamlines:

[chrishavlin]

@matthewturk this is ready for review I think! One part I wasn't sure about -- for the CurveCollection, I ended up using GL_LINES for plotting, which necessitates doubling the vertex data. I chose that over multiple opengl calls to draw multiple lines, but maybe there's a better approach here. Maybe OK for now, but certainly could be improved I think.