Up until now it was possible to store instances of `Node` subclasses
that do not have a registered entry point. Imagine for example the
definition of a `Data` subclass called `SubClass` in a shell and an
instance being stored. The node type string will be:

  `__main__.SubClass.`

When trying to load this node at a later point in time, the type string
can of course not be resolved to an importable class and the loader
would raise an exception.

The first change is that this exception is now turned into a warning and
the loader falls back onto the `Data` class. Note that we do not use the
`Node` class for this as this base class is also not storable and so the
ORM logic is potentially ill-defined for instances of the base `Node`.

Secondly, we now add a check to `Node.store` to make sure that the class
corresponds to a registered entry point. If this is not the case, the
store method will raise `StoringNotAllowed`.

One unit test was deleted because its implementation was incorrect and
was not actually testing what it was intending to test. Besides intended
functionality is now covered by other tests.