Refactor: change type of 'trackDuration' to float

CHANGELOG.md

schemas/openapi-tank.json

src/aura_engine/scheduling/models.py

src/aura_engine/scheduling/programme.py

src/aura_engine/scheduling/utils.py

src/aura_tank_api/models/file.py

src/aura_tank_api/models/playlist_entry.py

tests/json/tank-api-v1-playlists-1.json

tests/test_scheduling_api_tank.py