Support for Let Expressions (#478)

* Cleaned up schema, adjusting some documentation and grouping constraint constructs in a section of the schema.
* Added support for declaration of let expressions in constraints.
* Added documentation about the let expression in the specification.
* Added let example to specification.
* Fixed some other inconsistencies in the Metaschema XML schema.

examples/computer-example.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <METASCHEMA xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
- xsi:schemaLocation="http://csrc.nist.gov/ns/oscal/metaschema/1.0 ../../../schema/xml/metaschema.xsd"
+ xsi:schemaLocation="http://csrc.nist.gov/ns/oscal/metaschema/1.0 ../schema/xml/metaschema.xsd"
  xmlns="http://csrc.nist.gov/ns/oscal/metaschema/1.0">
     <schema-name>Computer Model</schema-name>
     <schema-version>0.0.5</schema-version>

schema/xml/metaschema-markup-line.xsd

@@ -3,7 +3,6 @@
 
   <xs:include schemaLocation="metaschema-prose-base.xsd"/>
 
-
   <xs:complexType name="MarkupLineDatatype" mixed="true">
     <xs:complexContent>
       <xs:extension base="inlineMarkupType"/>