refactor stage 1: data model for bisections
Model a bisection and a bisection set in a separate module that will later be useful for storage purposes and for modelling relationships between bisections, regressions and test runs.
Refactor the code in run_tracker.py to use the data model and add more details to the bisections template.
Signed-off-by: Ricardo Cañuelo ricardo.canuelo@collabora.com