Adjustment for YamlConverter to specify SimpleType / ComplexTypeSimpleCo...

# Issue
I try to convert the xsd files provided for the [VDV EKAP 431](https://www.vdv.de/ip-kom-oev.aspx) ([xsd archive](https://www.vdv.de/trias-xsd-v1.0.zipx?forced=true)) service using xsd2php.
This seems to work pretty well but when creating the JMS YAML metadata the output seems to be inconsistent to the generated classes.

Here's an example:
Generated class <code>Trias\LocationRefStructureType</code> types stopPointRef clearly as string:
```php
/**
 * Class representing LocationRefStructureType
 *
 * Reference to a stop, address, POI, locality
 * XSD Type: LocationRefStructure
 */
class LocationRefStructureType
{

    /**
     * @property string $stopPointRef
     */
    private $stopPointRef = null;

    /**
     * Sets a new stopPointRef
     *
     * @param string $stopPointRef
     * @return self
     */
    public function setStopPointRef($stopPointRef)
    {
        $this->stopPointRef = $stopPointRef;
        return $this;
    }
```

Generated metadata <code>Trias/LocationRefStructureType.yaml</code> types stopPointRef clearly as <code>Trias\StopPointRef</code>:
```yaml
Trias\LocationRefStructureType:
    properties:
        stopPointRef:
            expose: true
            access_type: public_method
            serialized_name: StopPointRef
            xml_element:
                namespace: trias
            accessor:
                getter: getStopPointRef
                setter: setStopPointRef
            type: Trias\StopPointRef
```

Used code to generate code:
```
php vendor/goetas/xsd2php/bin/xsd2php.php convert:php trias-xsd-v1.0/Trias.xsd \
 --ns-map='trias;Trias/' \
 --ns-dest='Trias/;src/Trias' \
 --ns-map='http://www.siri.org.uk/siri;Siri/' \
 --ns-dest='Siri/;src/Siri' \
 --ns-map='http://www.ifopt.org.uk/ifopt;Ifopt/' \
 --ns-dest='Ifopt/;src/Ifopt' \
 --ns-map='http://www.ifopt.org.uk/acsb;Ifopt/Acsb/' \
 --ns-dest='Ifopt/Acsb/;src/Ifopt/Acsb' \
 --ns-map='http://datex2.eu/schema/1_0/1_0;Datex2/' \
 --ns-dest='Datex2/;src/Datex2'
```

Used code to generate metadata:
```
php vendor/goetas/xsd2php/bin/xsd2php.php convert:jms-yaml trias-xsd-v1.0/Trias.xsd \
 --ns-map='trias;Trias/' \
 --ns-dest='Trias/;src/Trias' \
 --ns-map='http://www.siri.org.uk/siri;Siri/' \
 --ns-dest='Siri/;src/Siri' \
 --ns-map='http://www.ifopt.org.uk/ifopt;Ifopt/' \
 --ns-dest='Ifopt/;src/Ifopt' \
 --ns-map='http://www.ifopt.org.uk/acsb;Ifopt/Acsb/' \
 --ns-dest='Ifopt/Acsb/;src/Ifopt/Acsb' \
 --ns-map='http://datex2.eu/schema/1_0/1_0;Datex2/' \
 --ns-dest='Datex2/;src/Datex2' \
 --alias-map='http://www.w3.org/2001/XMLSchema;anyType;Drupal\clients_vdv\Jms\Handler\VdvAnyType' \
 --alias-map='http://www.w3.org/2001/XMLSchema;anySimpleType;Drupal\clients_vdv\Jms\Handler\VdvAnySimpleType'
```

# Solution proposal

I think the both types should match and since I think it's desirable to keep the nesting as low as possible the metadata should follow the generated classes and accept the simple type.
The changes I made check if the element is an instance of <code>SimpleType</code> / <code>ComplexTypeSimpleContent</code> and if so uses the inner type for the YAML type.
The example case now outputs this JMS YAML metadata:
```yaml
Trias\LocationRefStructureType:
    properties:
        stopPointRef:
            expose: true
            access_type: public_method
            serialized_name: StopPointRef
            xml_element:
                namespace: trias
            accessor:
                getter: getStopPointRef
                setter: setStopPointRef
            type: string
```