Make interface for BatteryPool metrics consistent with power methods

[sahas-subramanian-frequenz]

The soc, capacity, power_bounds methods of the BatteryPool
were directly returning receivers. They've now been converted to
properties, from which new_receiver() has to be called, to get a
receiver.

New:

soc_recv = battery_pool.soc.new_receiver()

Old:

soc_recv = battery_pool.soc()

[Marenz]

Merging is blocked
The base branch requires all commits to be signed.

[shsms]

commits are now signed, also added a release notes entry.

[Marenz]

syntax should be ```python

And this makes me realize that we aren't testing the RELEASE_NOTES examples yet

[Marenz]

@leandro-lucarella-frequenz do we want to test that? If so this example would fail of course

[leandro-lucarella-frequenz]

Mmmm, not sure about RELEASE_NOTES, sometimes we want to use old syntax/features to show an upgrade path, I would leave it outside.

[Marenz]

Well, that could be solved by either commenting the wold syntax/way or by having an extra ``` wrapped block that is not marked as python

[shsms]

removed the extra space.

[leandro-lucarella-frequenz]

I think that adds more noise than it helps. Examples in the RELEASE NOTES are a one-off thing, if you checked the time you write them, then they don't need to be kept up to date ever again. I really think that would make writing release notes more annoying and harder to read with very little gain.

[shsms]

Rebased, resolved conflicts.