Make dual mode GA-specific

[utensil]

See #425 (comment) . It's good in general but has no particular use case in the near future and it breaks compatibility.

[eric-wieser]

We may also want to change this API to help with #196 - having separate spellings for getting and setting, val = Ga.dual_mode_value and Ga.dual_mode(val), is a waste of names.

[eric-wieser]

One way we could perhaps handle that is a class like:

class PropertyAndClassMethod(property):
    def __init__(self, get=None, set=None, del_=None, doc=None, call=None):
        super().__init__(get, set, del_, doc)
        self.fcall = classmethod(call)

    def classmethod(self, f):
        return PropertyAndClassMethod(self.fget, self.fset, self.fdel, self.__doc__, f)

    def __get__(self, obj, owner):
        if obj is None:
            return self.fcall.__get__(obj, owner)
        else:
            return super().__get__(obj, owner)

class Ga:
    _dual_mode = 'default'
    @PropertyAndClassMethod
    def dual_mode(self):
        return self._dual_mode
    @dual_mode.setter
    def dual_mode(self, value):
        self._dual_mode = value
    @dual_mode.classmethod
    def dual_mode(cls, value):
        deprecate()
        cls._dual_mode = value

This will keep Ga.dual_mode("I+") working, but will break o3d.dual_mode("I+") with a TypeError.

[utensil]

It looks good to me except the name could be something without a "and" and more general, like ExtendableProperty and class method is optional and we could integrate cached_property together as one of the extendable functionalities.