Interval - make optional and respond with interval

[Alvie]

• remove interval_spacing from required args list
• only assign INTERVAL_SPACING if in content
• if not in content, set INTERVAL_SPACING to be END_TIME - START_TIME in seconds
• set timeBoundaries to be in seconds instead of minutes
• return intervalSpacing in final_data

[Alvie]

Already in previous commit from @BenGaraffa so closing this pull request