v2.6.0 - registry cleanup, retrieve efficiency, tests

What's Changed

Extended the leftover registry cleanup functionality to also detect and remove empty battery devices (devices without any entities) in addition to stale entities.

Introduced a new action to retrieve the charge/discharge efficiency at a given power level, useful when custom efficiency curves are defined. It can be used to estimate the energy reserve needed to provide a certain amount of energy out, if the power output is known.

Introduced extensive tests to ensure proper working for future pull requests

Full Changelog: v2.5.0b...v2.6.0b