Fixes for Ubuntu 18.04

[Tuebel]

Hi, I had some issues getting the training to work on Ubuntu 18.04 and a recent PyTorch version which PointNet2 seems to require.

• Updated the instructions in the README and requirements
• Fixed some typos.
• Removed synthetic data in LineMod training. Or do you have a LineMod dataset available with the render folder?
• Replaced neupeak.utils.webcv2 with cv2
• Replaced AT_CHECK with TORCH_CHECK
• Replaced CyclicLR with the torch implementation. I hope that the parameters are chosen correctly. For example the torch implementation defaults cycle_momentum=True during the initialization which I changed to False since the optimizer would require a momentum to be set. Is it a good idea to use momentum?

[Tuebel]

Ah great, I didn't notice the raster_triangle