small cleanup:

• fix bug when a single controller_type is specified
• if the camera in a subplot is not orthographic, then adding new graphics to a PlotArea will not stack them along the z-axis by setting z_position. We assume the user wants to manage all 3 axes when the camera is perspective.
• simple.ipynb -> quickstart.ipynb
• clearer readme, should consolidate GPU, adapter, drivers, and system libs docs at some point...
• better error message when len(enumerate_adapters()) < 1

Adds these two small examples to quickstart 😄