Thermostat service should have supported step property

## Issue
Some thermostats allow for different setpoint steps than the default 0.5 degree. Currently, according to the `thermostat` service, there is no way of knowing that.

## Proposition
This should be noted in service property `sup_step`, just like for the `water_heater` service.
https://github.com/futurehomeno/fimp-api#service-props-6