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 universam1/iSpindel#336

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

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