Quantity: dunder methods

[lucascolley]

Initial discussion at https://github.com/quantity-dev/quantity-api/pull/1/files#r1943441155

[nstarman]

To help the transfer, this was the Q

What are the other dunder methods we should provide?

__ne__, __lt__, __le__, __gt__, __ge__, __add__, __sub__, __mul__, __matmul__, __truediv__, __floordiv__, __mod__, __divmod__, __pow__, shifts, &, ^, |, ...

[andrewgsavage]

I'm in agreement with @mhvk https://github.com/quantity-dev/quantity-api/pull/1/files#r1943523289

Required would seem comparisons, negation/pos. (+q and -q), addition/subtraction, multiplication/division, power, and absolute value.

In a minimal API, floor division and modulo should probably be optional, as they're little used and not all that obvious, since integers have not that much meaning (conversely, if we include them, we have to define precisely what they are supposed to do [including perhaps what it means to floor divide by a unit]; we then should also include divmod and maybe round, floor, and trunc).

Shifts and bitwise operators should definitely not be required.

Anyway, is there anything wrong with doing what float has except removing things that have to do with integers like floor division, modulo, rounding, etc.? (Just to be sure, I mean remove from the required dunder methods for protocol purposes; we could list them as preferably present, though as I noted, then we have to be sure we agree on how they should be implemented too.)

[nstarman]

Agree on the necessity of __ne__, __lt__, __le__, __gt__, __ge__, __add__, __sub__, __mul__, __matmul__, __truediv__, __pow__!

And on shifts and bitwise operators. For ArrayQuantity we can look to the set of methods defined on the Array API.