Implement an abstract base class for Quantity

[nstarman]

Concrete classes generally need to implement class-specific logic. Having good ABC structures underpinning the concrete classes makes defining new concrete subclasses significantly easier.

class AbstractQuantity(ABC): ...

@final
class Quantity(AbstractQuantity): ...

class AbstractAngle(AbstractQuantity): ...  # still an ABC

@final
class Longitude(AbstractAngle): ...

@final
class Latitude(AbstractAngle): ...

[mhvk]

In principle, definitely OK with me. There just didn't seem much of a benefit yet, and I wanted to have something that at least did the basics!