heater.c: Limit PID I term with conditional integration.
heater.c: Enable more anti-windup with PID_CONDITIONAL_INTEGRATION.
Merge branch 'issue74-ConditionalIntegration' into experimental
heater.c, config.default.h: Make PID_CONDITIONAL_INTEGRATION non-optional per https://github.com/Traumflug/Teacup_Firmware/issues/74#issuecomment-38999466