RTD conversion

[Jakeler]

Extending on #18: add PT100, PT500, PT1000 resistance temperature detectors (RTD) Settings:

• RTD mode
  • Calculate from resistance temperature detectors (PT100, PT500, PT1000 ...)
• Nominal resistance @ 0 °C
• Alpha factor [Ω/(Ω·°C)]

[Jakeler]

Maybe B,C term? https://en.wikipedia.org/wiki/Resistance_thermometer#Standard_resistance_thermometer_data Fixes slight non linearity. Ask manufacturer