Use dir() instead of vars() in Referable.update_from()

[zrgt]

vars() only retrieves the instance's
dict—that is, the object's
attributes—but does not include
properties.
As a result, when you use vars(other),
you get the _id_short attribute instead
of the id_short property.

dir() iterates over all attributes and
properties of the object.
We skip callables and private attrs.
We use setattr() to set attributes and
properties, so setter funcs will be
used.

Fixes #215

[zrgt]

The question here is: how should we handle properties that are not permitted to be set, such as SubmodelElementList.order_relevant?

In the previous solution, we simply set the attribute, even though it was not allowed. However, with the vars() and setattr() approach, an exception is raised, indicating either that no setter exists for the property or that setting it is explicitly forbidden.