Re #6490. Put back SNS <archive> tag.

This is required for the archive search within the FileFinder.
Although it uses ICAT underneath, it is unrelated to the Catalog menu.

Code/Mantid/instrument/Facilities.xml

@@ -263,6 +263,11 @@
 
 <facility name="SNS" delimiter="_" FileExtensions="_event.nxs,.nxs.h5,.nxs,.dat,_runinfo.xml,_histo.nxs">
 
+   <archive>
+      <archiveSearch plugin="SNSDataSearch" />
+      <archiveSearch plugin="SNSDataSearchICAT2" />
+   </archive>
+
    <instrument name="DAS"> 
       <technique>technique</technique>
    </instrument>