[repository schema] Make all element types optional

[rshriver]

It is proposed to remove the requirement for any root level element type to be present. This affects datatypes, fields, and messages. For example, an Orchestra XML file may only contain group and component definitions but no messages.

<xs:element name="repository">
  <xs:complexType>
    <xs:all>
      <xs:element name="metadata" type="dcterms:elementOrRefinementContainer"/>
      <xs:element ref="fixr:categories" minOccurs="0"/>
      <xs:element ref="fixr:sections" minOccurs="0"/>
-     <xs:element ref="fixr:datatypes"/>
+     <xs:element ref="fixr:datatypes" minOccurs="0"/>
      <xs:element ref="fixr:codeSets" minOccurs="0"/>
-     <xs:element ref="fixr:fields"/>
+     <xs:element ref="fixr:fields" minOccurs="0"/>
      <xs:element ref="fixr:actors" minOccurs="0"/>
      <xs:element ref="fixr:components" minOccurs="0"/>
      <xs:element ref="fixr:groups" minOccurs="0"/>
-     <xs:element ref="fixr:messages"/>
+     <xs:element ref="fixr:messages" minOccurs="0"/>
      <xs:element ref="fixr:concepts" minOccurs="0"/>
      <xs:element ref="fixr:scenarios" minOccurs="0"/>
      <xs:element name="annotation" type="fixr:annotation" minOccurs="0"/>
    </xs:all>
    ...