Implement an abstract base class for Quantity

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

```python
class AbstractQuantity(ABC): ...

@final
class Quantity(AbstractQuantity): ...

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

@final
class Longitude(AbstractAngle): ...

@final
class Latitude(AbstractAngle): ...
```

The issue names no files or tests. Start by locating Quantity and its concrete subclasses, then review how class-specific logic is currently organized. Done means the requested abstract hierarchy exists, Quantity and the listed concrete classes have the stated relationships, and the abstract classes remain usable as ABCs.

Written by the indexing model from the issue text.