AT2-797 add support for scan_duration to LOW TMCConfiguration
This MR adds support for LOW to the TMCConfiguration.
With scan duration as the only class property, there's no way for the schema to infer whether it's targeting SKA MID or SKA LOW. Rather than having scanDuration and scan_duration, I chose to give the TMCConfiguration a single scan_duration property and an 'is_ska_mid' property, which the schema uses to determine if it should output LOW or MID JSON. When we make everything ADR-35 compliant the is_ska_mid property can be deleted.