GT3X: Actfast returns uncalibrated sensor data

[ReinderVosDeWael]

As reported in childmindresearch/wristpy#87 , there may be an issue with the lux processing. This was asked here: actigraph/GT3X-File-Format#29 , but to this date there has not been a response.

The current implementation is based on this.

[nx10]

Unless the manufacturer provides an updated calibration value table this will be [wontfix]. (Better to report uncalibrated data than wrong data)

It's possible that these sensors are not calibrated at all and the user is expected to measure their own calibration curve per device.

[nx10]

It might however be worth it to provide indicators about units for all measurements so this can be handled programmatically downstream.