Rename `xxx_pool()` functions to `new_xxx_pool()`

[llucax]

What's needed?

We need to make it more clear that functions like battery_pool() are not really used to get preexisting pools, but to create new ones.

Proposed solution

Rename xxx_pool() to new_xxx_pool() in frequenz.sdk.microgrid (and _DataPipeline).

Use cases

No response

Alternatives and workarounds

No response

Additional context

• Consider different approaches to power management priorities #913
• New `microgrid` logical components and the role of `logical_meter` #782