Status endpoint last calibration time should pull from signal analyzer

[aromanielloNTIA]

Currently the status endpoint loads the sensor cal on SCOS Sensor startup, and reads the last_calibration_time directly from this.

Since Y-factor calibration is implemented as an action, the status endpoint should query the signal analyzer for the most recent sensor cal time.

The best approach here is probably the keep the existing field, and add a new one: last_onboard_calibration_time or last_field_calibration_time. This way the status endpoint indicates the date from the lab-provided calibration data as well as any in-field calibrations performed onboard.

Relevant code here