DS-1262: XMLUI - export metadata from search results

I have improved upon the method of 'exporting' metadata to a csv as in it does this after selecting Export Metadata from the Context option. Not during the search. The sitemap processes the request through new class SearchMetadataExportReader in the cocoon package. In order to utilize the non instantiable abstract class AbstractSearch and its protected DiscoverResult variable queryResults, I copy the protected variable into a private static variable after a search is performed. This allows a static method to access it.

https://jira.duraspace.org/browse/DS-1262