Profile specification (usnistgov#1017)

* Updated readme in utils/util (usnistgov#1014)
* First draft of profile resolution specification rework
* Integrated feedback from 8/31 meeting w/Dave and Wendell
* Removed of Explicit/Implicit selection. Added last examples, Target section cleanup
* Resolves usnistgov#1046 usnistgov#1043 usnistgov#1042 usnistgov#1041 usnistgov#1040 usnistgov#1039
* Added "flat" 
* Update specml-html-preview.xsl

Co-authored-by: David Waltermire <david.waltermire@nist.gov>
Co-authored-by: Wendell Piez <wendell.piez@nist.gov>
Co-authored-by: Stephen Banghart <stephen.banghart@outlook.com>

src/metaschema/oscal_profile_metaschema.xml

@@ -92,6 +92,7 @@
                   <assembly ref="combine"/>
                   <choice>
                         <field ref="as-is"/>
+                        <assembly ref="flat"/>
                         <assembly ref="custom"/>
                   </choice>
             </model>
@@ -113,6 +114,10 @@
             <formal-name>As is</formal-name>
             <description>An As-is element indicates that the controls should be structured in resolution as they are structured in their source catalogs. It does not contain any elements or attributes.</description>
       </define-field>
+      <define-assembly name="flat">
+            <formal-name>Flat</formal-name>
+            <description>A Flat element indicates that the controls should be structured in a completely flat list. It does not contain any elements or attributes.</description>
+      </define-assembly>
       <define-flag name="method" as-type="string">
             <formal-name>Combination method</formal-name>
             <description>How clashing controls should be handled</description>