Add optional MHK Fahrenheit correction mode

[mmeisel]

This PR closes #33 (as best as can be done, it seems). See the discussion there, but in summary:

• Add an optional configuration option mhk_fahrenheit_correction which defaults to false. When set to true, it will attempt to adjust any temperatures exchanged with the MHK such that the display will show an accurate Fahrenheit value.
• This alters both set points and current temperatures (but see caveats below).

Caveats:

• This requires Allow setting temperatures on get packets itp-packet#10
• This will always work for set points, but will only work for the current temperature when the MHK is using an external temperature source. (In the installer menus, ISU 190 needs to be set to "Indoor Unit" or presumably one of the RedLINK options if available, though I haven't tested this.) When the thermostat is using its internal temperature source, it seems to ignore anything we send it and always use its internal Fahrenheit conversion.
• The lookup table is based on the one in the project documentation and the one in echavet's firmware. I have not had time to test that the look up table is 100% correct. Ideally, it would be good to create a virtual temperature source that can be set to output each temperature in turn so that we can make sure the display shows the expected value.