Missing imports

[NoahRJohnson]

Some vxl core libraries interact with each other. This causes issues when importing only a single library. For example,
from vxl import vpgl
cam = vpgl.load_perspective_camera('/path/to/camera.txt')
print(cam.camera_center)
errors, because the camera_center property of a vpgl_perspective_camera is a vgl_point_3d.

How should we solve this? Make a list of all possible interactions, and then add the imports to each library's __init__ file?