Introduce segmentation handler

[chingyulin]

This PR introduces a new IO module for segmentation. Specifically, a metaclass BaseSegmentationHandler and a class NumpySegmentationHandler are introduced. This module paves the way for enabling segmentation mode. See https://github.com/ch-sa/labelCloud/pull/87 for more details and how this module is intended to be used.

Note that I opted for implementing its own module in stead of inheriting the BaseLabelFormat like you originally suggested @ch-sa because the fundamental differences between the box annotations and the point annotations. For example, the BBox in the BaseLabelFormat.import_labels and BaseLabelFormat.export_labels are not relevant for segmentation label; and the label_definition (schema), array representation of the segmentation label are not relevant for the box labels. I'm open for different suggestions tho.