Quantity: add dunder methods

[andrewgsavage]

transferred from quantity-dev/quantity-api#6

[lucascolley]

I've added some tentative suggestions but I'm not sure that they are correct — @jorenham do my suggestions look closer to what we would want here?

[andrewgsavage]

I feel like we need to get a type checker going to know what works and doesnt

[lucascolley]

I feel like we need to get a type checker going to know what works and doesnt

Yes, but we then also need a minimal an implementation to check against :)

[andrewgsavage]

I feel like we need to get a type checker going to know what works and doesnt

Yes, but we then also need a minimal implementation to check against :)

does it have to be minimal? would a fork of pint (or another module) work?

[lucascolley]

would a fork of pint (or another module) work?

yeah, I think so!

[jorenham]

I've added some tentative suggestions but I'm not sure that they are correct — @jorenham do my suggestions look closer to what we would want here?

My mental typechecker did not report any errors 👌🏻

[jorenham]

I feel like we need to get a type checker going to know what works and doesnt

Exactly; writing annotations without a type-checker is like programming without a compiler or interpreter

[nstarman]

When would R in __pos__ ever not be V? I can't think of any examples. 🧐

[jorenham]

>>> +True
1