Area for Region ontology

**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**
Add any other context or screenshots about the feature request here.

The issue names no files or tests; first locate the Region and Box ontology definitions and inspect how a Region exposes its represented image or enclosed pixels. Confirm the existing binary-image behavior, then add area coverage that counts the pixels enclosed by a Region and run the relevant ontology tests.

Written by the indexing model from the issue text.