Enable longterm statistic

[kuzabi]

Hello,
first of all, great job, it's work like a charm.

I've proposition to enable long term statistic for sensors.
As far as I understand there should be defined a "state_class: measurement" for an sensor entity and that should do the job.
https://developers.home-assistant.io/docs/core/entity/sensor/#available-state-classes

Currently is need to prepare template sensor with defined "state_class" based on Hydro sensor.

It could be as optional parameter in sensor configuration.
My programming skills are not on that level to be able to provide a code proposition - sorry.
I think it could be valuable for some users and it seems not so complicated to implement.

[PiotrMachowski]

Added in v1.0.7