Add support for building feature extraction

[issa-tingzon]

Six different types of buildings: (1) residential, (2) damaged, (3) commercial, (4) industrial, (5) education, (6) health.

For each lat-lng and specified radius or bounding box, extract the following features:

• [x] total number of buildings
• [ ] the total area of buildings
• [ ] the mean area of buildings
• [ ] proportion of area occupied by the buildings