Explore if Quantity can be made an abstract class

[llucax]

Coming from:

• Add Quantity types and update formulas to produce typed Samples #422 (comment)

Maybe we make Quantity named something like BaseQuantity and make it an ABC and then have Quantity = Power | Current | Voltage | Energy | ....

Of course the downside of this is that if we add new units, we need to update the Quantity type alias, but for me it seems a bit confusing and counterintuitive to have a unit-less quantity.

@shsms

The reason it it like this is because in cases like the FeatureExtractor, we need a fallback type that everything would decompose into. So we pass any receiver to the FeatureExtractor, and it would be treated as a Quantity. Because from that point we'll only deal with numpy arrays, for example, we can't propagate the types downstream.

@llucax

I still don't understand why you need the unit-less Quantity type, if you need it as a step to pass it to numpy or something like that, then you can decompose into a float already, right?

[llucax]

@shsms (#422 (comment))

It turns out MovingWindow etc can't propagate types because the most likely output types for these classes is a numpy array. So it doesn't have to be generic, just needs to accept any thing that gets put in here. And using the base Quantity type achieves that.

Wouldn't it make more sense then to just keep MovingWindow and anything that doesn't care about units as float then, and have these classes unwrap the float inside the Sample value Quantity?

[llucax]

There is an attempt in #822. I hit a wall when reaching the formula engine because the typing in there is a bit messed up (probably because it was done when mypy was not running). The formula engine will probably need a big revamp before being able to make quantity an abstract base class.