Add sensor for return temperature at year scale

This release adds one new sensor with the value of the current year average return temperature and attributes with further data from earlier years as well as other numbers for temperatures, M3 and energy at year scale.

This extra data could also be put in separate sensors, but I did not find a use for it right now.

The release implements #38

Bugs etc. at Github as usual. If you encounter bad things it is safe to downgrade.

Full Changelog: v1.0.6...v1.1.0