Area for Region ontology

[hepengfe]

Is your feature request related to a problem? Please describe.
Current region doesn't have area function that computes area of the represented partial image.

Describe the solution you'd like
Unlike box that has a regular shape that can be computed from height and width, region as a more generic CV object could be regular or irregular. Therefore, region should be computed by counting the number of pixels enclosed by the region object. For example, for binary image, area of region is the number of 1.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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