pythreejs loads unnecessarily

When using `compas_viewer` (which seems to be improving a lot...) I noticed that an unrelated module it loaded (pythreejs, related to `compas_notebook`). Loading an unrelated library (I've worked with pythreejs quite a bit, its pretty slow to load) could hinder a faster startup time of `compas_viewer`

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/compas_libigl/docs/examples/example_geodistance_multiple.py
None
**PyThreeJS** Model elements registered.
**PyThreeJS** SceneObjects registered.
Color(red=0.154, green=0.680, blue=0.504, alpha=1.000)
Color(red=0.304, green=0.765, blue=0.420, alpha=1.000)
Color(red=0.142, green=0.526, blue=0.556, alpha=1.000)