removed get_duration() from Incident model and updated model unit tests accordingly
Custom Manager for Incident Model

- created IncidentManager a custom manager for Incident
- factored out aggregate call into avg_units_by_type()
- test case for avg_units_by_type() function
adding avg_units_and_duration_by_level() function to Incident Manager
adding descriptions to the charts in incident view
avg_units_by_type() groups incident type up until the first hyphen

- treats the incident type 'Type 1 - thing' as 'Type 1'
- updated unit test reflect the change
updates to Incident view

- update figure 1 blurb
- resized width of figure 1
- some styling and wording changes
Updating README with installation instructions