Multi MPPT changes for monophase inverters

This release contains changes for monophase + multi MPPTs and restructures the sensors definition to make it more flexible.
It should solve issue #69 (wrong DC value of -0.0 for monophase + multiple MPPTs setups).

If you have a monophase inverter with multiple strings, please provide a feedback in Discussions section, every release can be discussed in the Announcements category. You can also give the feedback in HA forums thread.

Thanks a lot to @ivanfmartinez for his contribution.