Split the `Quantity`s to its own repository

[llucax]

What's needed?

We need to allow cloud applications to be able to use Quantitys without depending on the microgrid. We might also want to use Quantitys in API clients too, and they should definitely not depend on the SDK.

Proposed solution

Split the Quantity and all its sub-classes to its own repository:

• Create the new repository frequenz.quantity and move the code there
• Release a v1.0.0 version (we want to ensure stability from day one, as we are stabilizing all the base repos).
• Remove the code from the SDK and depend on the new repo

Use cases

All the cloud apps, maybe all API clients.

[llucax]

frequenz-floss/frequenz-quantities-python#7