Create gravity calibration

[ckrack]

Support for a calibration based on submitted data would be great.

Much like here: https://universam1.github.io/iSpindel/tools/calibration/calibration.htm
But based on actual data-points.

Scenario:
Select data-points in the list.
Define the measured value for them in a textinput.
These points will be saved to the database (calibrations_points(id, datapoint_id, correct_value)).
The calibration is saved as polynomial values in it's own table (calibrations).