[repository schema] New element for encoding information specific to a datatype mapping

[kleihan]

Datatypes already have a limited ability to add encoding information with the mappedDatatype element, whereby the scope of this element is a single encoding standard. It is proposed to replace the extension element with a new encoding element inside mappedDatatype to support the full encoding flexibility for a given datatype mapping. It is further proposed to remove the simple types datatypeStandard_t and datatypeStandard_enum and redefining element standard as a token limited to 64 characters (type Name_t).

<xs:complexType name="mappedDatatype">
  <xs:sequence>
    <xs:element name="encoding" type="fixr:unnamedEncoding" minOccurs="0"/>
    <xs:element name="annotation" type="fixr:annotation" minOccurs="0"/>
  </xs:sequence>
  <xs:attribute name="standard" type="fixr:Name_t" use="required"/>
  ...
</xs:complexType>

The element unnamedEncoding is defined by issue #250.