Structured Node `__eq__` works incorrectly for non-persisted nodes.

Expected Behavior (Mandatory)

If nodes are not created in the database and do not have an element_id property, they should be compared by id in memory or always count as different.

Actual Behavior (Mandatory)

All non-persisted node countries as equal

How to Reproduce the Problem

Create any StructuredNode objects without saving them to the database. Call comparison.

Simple Example

from neomodel import StructuredNode

class Foo(StructuredNode):
    pass

a = Foo()
b = Foo()

print(a == b)

The problem comes from the StructuredNode.__eq__ function.

    def __eq__(self, other):
        if not isinstance(other, (StructuredNode,)):
            return False
        if hasattr(self, "element_id") and hasattr(other, "element_id"):
            return self.element_id == other.element_id
        return False

I haven't prepared a pull request because I'm unsure which behavior should be selected as a default: all non-persisted nodes. Are they not equal, or do we want to compare them by memory ID or the other way around?

neo4j-contrib / neomodel