BPMN2 — tActivityResource::resourceRef should be optional

##Source: IBM (Matthias Kloppmann, matthias-kloppmann@de.ibm.com)
##Original Issue: http://www.osoa.org/jira/browse/BPMN-494
##Original Info: (Severity: Significant - Nature: Revision)

tActivityResource::resourceRef is a required attribute, but should be optional - in case the resource is resolved via a ResourceAssigmnentExpression there won't be any statically assigned resourceRef (except for a dummy, which is how I resolved the problem in the example).

<performer resourceRef="dummy">
<!-Issue: If specified via an expression, a resourceRef is not needed, i.e., the attribute must not be required.->
<resourceAssignmentExpression>
<formalExpression>getInputData()/resource</formalExpression>
</resourceAssignmentExpression>
</performer>

(a) Replace Figure 10.7 (pg 161)
Update the UML metamodel, giving the ActivityResource.resourceRef association a multiplicity of [0..1]
(b) Update Table 10.5 (pg 162)
Specify a multiplicity of [0...1] for the resourceRef association
New description for resourceRef: The Resource that is associated with the Activity. Should not be specified when a resourceAssignmentExpression is provided.
Append to description for resourceAssignmentExpression: Should not be specified when a resourceRef is provided.
Append to description for resourceParameterBindings: Is only applicable if a resourceRef is specified.
(c) Update Table 10.30 (pg 175)
Replacement XSD snippet
<xsd:complexType name="tActivityResource">
<xsd:complexContent>
<xsd:extension base="tBaseElement">
<xsd:choice>
<xsd:sequence>
<xsd:element name="resourceRef" type="xsd:QName"/>
<xsd:element ref="resourceParameterBinding" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:element ref="resourceAssignmentExpression" minOccurs="0" maxOccurs="1"/>
</xsd:choice>
</xsd:extension>
</xsd:complexContent>
</xsd:complexType>
[Note: a consequence of formalizing in the XSD as a 'choice' is that the resourceRef is now an element instead of an attribute]
(d) Update Table 10.19 (pgs 181-182)
Replace the 'resourceRef' attributes with equivalent 'resourceRef' elements.
Disposition: Resolved