XSD-Schema: group `qualifiable` definition inelegant and confusing
The current Schema uses the plural "s" for qualifiers in group `qualifiable` in a confusing way. For example a qualifiable property could look like this, when serialized according to the current schema:
```
<property>
[...]
<qualifier>
<qualifiers>
<qualifier/>
</qualifiers>
<qualifiers>
<formula/>
</qualifiers>
[...]
</qualifier>
[...]
</property>
```
The wrapping `<qualifiers>` element around each inner `<qualifier>` or `<formula>` seems unneccesary. We suggest to remove it, to look like this:
```
<property>
[...]
<qualifiers>
<qualifier/>
<formula/>
[...]
</qualifiers>
[...]
</property>
```
This can be achieved by changing the current version of the XSD-Schema from:
```
<group name="qualifiable">
<sequence>
<element maxOccurs="1" minOccurs="0" name="qualifier" type="aas:constraints_t"/>
</sequence>
</group>
<complexType name="constraints_t">
<sequence>
<element maxOccurs="unbounded" minOccurs="0" name="qualifiers" type="aas:constraint_t"/>
</sequence>
</complexType>
<complexType name="constraint_t">
<choice>
<element maxOccurs="1" minOccurs="0" name="formula" type="aas:formula_t"/>
<element maxOccurs="1" minOccurs="0" name="qualifier" type="aas:qualifier_t"/>
</choice>
</complexType>
```
to our suggested fix:
```
<group name="qualifiable">
<sequence>
<element maxOccurs="1" minOccurs="0" name="qualifiers" type="aas:constraints_t"/>
</sequence>
</group>
<complexType name="constraints_t">
<sequence>
<group maxOccurs="unbounded" minOccurs="0" ref="aas:constraintGroup"/>
</sequence>
</complexType>
<group name="constraintGroup">
<choice>
<element maxOccurs="1" minOccurs="0" name="formula" type="aas:formula_t"/>
<element maxOccurs="1" minOccurs="0" name="qualifier" type="aas:qualifier_t"/>
</choice>
</group>
```
issue