Refactor schema files to use imports and extensions of previous schemas

[jeanetteclark]

Similar to #489, we should refactor the xsd docs to repeat less code so they are easier to maintain and it is easier to see the changes between them. The crux of the issue is getting something like this to work:

 <xs:import namespace="https://nceas.ucsb.edu/mdqe/v1" schemaLocation="schema1.xsd" />
 <xs:import namespace="https://nceas.ucsb.edu/mdqe/v1.1" schemaLocation="schema1.1.xsd" />

  <xs:complexType name="selector">
    <xs:complexContent>
      <xs:extension base="v1:selector">
        <xs:sequence>
          <xs:element name="xpath" type="tns:expression" minOccurs="0" maxOccurs="unbounded"></xs:element>
          <xs:element name="expression" type="tns:expression" minOccurs="0" maxOccurs="unbounded"></xs:element>
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>

Currently, it is not working. Possibly related to #488 but I'm still not sure what the path forward is here.