Remove return_obj from Entity parse/serialize/transform methods

[bworrell]

This would need to carry through to python-stix|cybox|maec. We don't need it except in places where we have extension points.

We have code that passes around a return_obj to handle type inheritance in to/from dict/obj methods.

class Foo(Entity):
    _binding_class = some.binding.classname

    def to_obj(self, return_obj=None, ns_info=None):

        if return_obj is None:
            return_obj = self._binding_class()

        return_obj.value = self.value
        return return_obj

class DerivedFoo(Foo):
    _binding_class = some.other.binding.classname

    def to_obj(self, return_obj=None, ns_info=None):
        if return_obj is None:
            return_obj = self._binding_class()

        # Apply super attributes to the return_obj we created here
        super(DerivedFoo, self).to_obj(return_obj=return_obj, ns_info=ns_info)

        # Apply our own attribtes and return
        return_obj.foo = "bar"
        return return_obj

But it could be replaced with code that makes better use of super().

class Foo(Entity):
    _binding_class = some.binding.classname

    def __init__(self):
        self.value = "value"

    def to_obj(self, ns_info=None):
        obj = self._binding_class()
        obj.value = self.value
        return obj


class DerivedFoo(Foo):
    _binding_class = some.other.binding.classname

    def __init__(self):
        super(DerivedFoo, self).__init__()
        self.bar = "bar"

    def to_obj(self, ns_info=None):
        obj = super(DerivedFoo, self).to_obj(ns_info=ns_info)
        obj.foo = self.foo
        return obj

Most of this would be in python-stix|cybox|maec, but the changes would start in mixbox.