engine/views:

Added `allowed_methods` decorator to control the allowed request types
(GET, POST, etc.) in a DRY way.
engine/views:

Added views for exporting risk and hazard results.
engine/views:

Added a missing decorator to `calc_risk`.
engine/views:

Added a bunch of missing docstrings.
engine/v1/calc_urls:

Added url mapping for `get_hazard_result` and `get_risk_result`.