Use absolute import for treemap.json_field

implicit relative imports are discouraged and removed in python 3. Explicit relative imports are allowed, but absolute imports are preferred.

http://www.python.org/dev/peps/pep-0008/#imports
Prefer updating context {} over reassigning
Hoist inner function to top for readability
Hide ecobenefits when not overlapping itree region

fixes #600 on github.

Hide ecobenefits for any instance that does not touch an itree region