This commit remanes `valueDataType_t` from the XSD to
`valueDataType`.
Furthermore, it adds a missing `<reference>` tag around
the `refersTo` References of `Extension`.

The current order of the elements in `SubmodelElementList`
was wrong. This updates the order.

Version 3.0 of the spec removes the attribute `kind` from
`SubmodelElement`s.
While we already implemented this, it was still missing in the
XSD Schema, as well as the examples. This commit fixes that.

Currently, the XML deserialization missed the
`<aas:refersTo>` wrapper around the single references
inside `Extension.refers_to`. This commit fixes that.

Currently, `Extension.refers_to` is declared as a `Iterable[Reference]`.
This implies, that we can not necessarily check, whether or not the
attribute is empty or not.
This creates a problem with the XML serialization, since the
`<aas.refersTo>` element should only appear if there is at least
one `Reference` inside.

This commit changes the `Extension.refers_to` to be a list of
`Reference`s, as well as adapting a more clear check whether
or not the attribute is empty in `adapter.xml.xml_serialization`.

extract methods

extract methods

extract methods

extract methods