Add a DecimalEnergy class

[llucax]

In frequenz-floss/frequenz-api-electricity-trading#17 we detected the need for a Quantity-like class that is backed by a Decimal.

Since we still don't know how common this would be, the easiest approach would be to just add a new DecimalEnergy class that has the same interface as Quantity but uses a Decimal as the underlying value, just for this use case, which seems to be pretty unique. In the future we'll re-asses if we need to merge it with Quantity or we need some other solution.

Since this is only used by the electricity trading API so far, this DecimalEnergy might just only be included in the electricity trading client instead of the SDK.

If possible, this new class could inherit from Quantity too.

Alternative solutions are:

• Use Decimal as a backing type for all Quantitys (too big of a change, too disruptive, we would need to asses what would be the impact in performance and other aspects for other parts of the system).
• Parametrize Quantity to have different backing types for values, so we could have Energy[float] and Energy[Decimal]. This feels like it will be a lot of problem and pollute 99% of the code that needs to use float for very little gain.

[llucax]

@camille-bouvy-frequenz @thomas-nicolai-frequenz FYI. Do you have any preferences opinions about where this DecimalEnergy should live? It could be in the SDK, but it feels a bit too niche for me, but maybe you know about more use cases outside of the trading API.

[thomas-nicolai-frequenz]

Well this is derived from the common API repo. So yes there will be more use cases outside of trading.