Configuration sanity checks, good defaults, something optional

[jpulakka]

Currently we require two settings

    nordpool_entity: sensor.nordpool_kwh_fi_eur_3_095_024
    filter_length: 5

If either of those is missing, we get clear error, but if they are present but wrong, unplanned things happen.

• How to check for existence of nordpool_entity entity? How is this done in built-in HA components, e.g. in https://www.home-assistant.io/integrations/filter/ ?
• Perhaps nordpool_entity could be made optional, and automatically find the entity based on its name or type? Or better explicit than implicit?
• Add sanity limits to filter_length, 2...10, and fallback to good default default, e.g. 5. Also could make filter_length optional.

Some help in https://developers.home-assistant.io/docs/development_validation/

[jpulakka]

Done in https://github.com/jpulakka/nordpool_diff/commit/3880ef3c60633044368b2719272e182ae27a47e0