Support SemanticKITTI dataset

[ZCMax]

This is a development requirement documentation of a feature we wish to collaborate with the community.
In this issue, we wish to welcome the community developers to participate in the support of SemanticKITTI dataset.

Steps

• Implement the SemanticKITTIDataset
• Add unit tests to test your code
• Add KITTI Dataset for 3D Semnantic Segmentation introduction in doc

Resources

• Offical paper: https://arxiv.org/abs/1904.01416
• Experimental API for reference in MMDet3D: https://github.com/open-mmlab/mmdetection3d/blob/master/mmdet3d/datasets/semantickitti_dataset.py

[Jacky-gsq]

@ZCMax Hi, I also want to pick this issue and have discussed the development plan with Wenwei Zhang. Thanks!