Add ability to calibrate a tilt and apply temperature correction

[duncan-brown]

This pull request adds the ability to calibrate a tilt based on the calibration code that @vitotai write for the iSpindle in https://github.com/universam1/iSpindel/pull/336

This pull request is a work in progress. Items to complete before merge are:

• [x] Add JavaScript to compute calibration coefficients.
• [x] Fix HTML to pass calibration coefficients to server.
• [x] Add function in C++ code that sets and implements calibration.
• [x] Add function in C++ code that clears the calibration.

The code also adds the ability to correct the gravity from the temperature measured by the tilt to a reference temperature of 60F.