Get EC code building again; add MAX31855 support (scons platform=... max...

...31855=true); add ATmega 328 vs. 168 build control (scons platform=... mcu=atmega328) -- support was sort of there but not complete; change PID so that PID state is only reset when the set point changes by more than +/- 10C; keep temp as float in PID & TemperatureSensor classes