Move constants to attributes

[ntessore]

The chance that an existing implementation matches the exact namespace and name of the defined constants is slim (I don't know any code that does). Therefore, it's fair to say all existing codes must do some wrapping of their constants to conform to the API. In which case it is much more straightforward for everyone to have the constants in the cosmo object itself:

class HasC(Protocol):
    @property
    def c(self) -> Any:  # noqa: ANN401
        """Speed of light in km s-1."""
        ...

class HasG(Protocol):
    @property
    def G(self) -> Any:  # noqa: ANN401
        """Gravitational constant G in pc km2 s-2 Msol-1."""
        ...

[nstarman]

I like introducing the HasX forms. It actually applies equally well to cosmologies that have it on the cosmology object as it does to any that have it on an enclosed namespace.
I think this is a good addition, regardless of whether the constants namespace is made less prominent.