Multiple sensors for consumption/production

[Mulando]

Currently under "consumption" or "production" only one sensor is allowed. It would be great if you could add multiple sensors and the UI shows the sum of those.

E.g. in my case, my inverter has sensors for all 3 power phases with the current values on each phase. But it does not have a sensor-value for all current values in one number.

[Mulando]

Just created a pull request to allow this by piping entity names.
#670