Rename GridFrequency to a more suitable name

[daniel-zullo-frequenz]

What's needed?

The name GridFrequency might be confusing as it somehow clashes with the grid connection Grid. or ComponentFrequency would be more suitable

Proposed solution

Rename GridFrequency to a more suitable name like MicrogridFrequency.

Related documentation and tests need to be updated.

Alternatives and workarounds

Alternatively rename it to ACFrequency, because there's only one value for one microgrid, whether it is connected to the grid or not.

Additional context

Originally posted in #758 (review)