reference `reftype` should be enforced

[Jean-Paul Calderone]

reftype doesn't do the most obvious thing it could do:

from axiom.store import Store
from axiom.item import Item
from axiom.attributes import reference

class X(Item):
    foo = reference()

class Y(Item):
    bar = reference(reftype=X)

class Z(Item):
    baz = reference()

def main():
    s = Store()
    print Y(store=s, bar=Z(store=s)).bar

if __name__ == '__main__':
    main()

Instead, I guess it does other things - like serve as documentation about what you *should* be putting here. Also it provides a deletion optimization (which ends up being incorrect if you disregard the documentation purpose and make the reference to a different type ...)

It seems Axiom should not allow assignment of the wrong type.