Add an explicit way to signal we don't want to set power for a battery pool anymore

[llucax]

Coming from #591 (comment)

For now we are accepting set_power(0) to say we don't want to set power for a battery pool anymore, even if the exclusion bounds says we can't set the power to 0 (the battery pool will accept 0 as a special case and forward it even if it is out of bounds).

This is not a very clear interface. When there are exclusion bounds that forbid 0 as a value, as an user it is confusing. If I want to stop using a battery but the bounds say I can't set power to 0, I still have to send a set_power(0). Maybe someone will just send the lowest power accepted by the bounds instead, thinking that they can't use 0.

It would be better to have an explicit way to say "release this battery(pool)".

Either set_power(None) or maybe even a different method, like battery_pool.release().

Then the implementation can still send 0 even if it is out of bounds, that's fine.

This might be implemented / solved in #161.

[llucax]

Closing as won't fix because with the introduction to the power manager this won't be necessary anymore.

When the power manager is introduced (#161), then battery_pool.set_power() will be replaced by battery_power.request_power() (or similar) which will always be best-effort and will do whatever is on the range of operation of that battery pool, so it will never fail if the requested power is out of bounds (instead it will be adjusted appropriately).

So in this case using request_power(0) is perfectly fine to signal we don't need more power from this battery pool.

We could still consider adding a release() method for extra clarity though.