improve explain operation

[guenterh]

http://localhost:8080/sru/search?query=&operation=explain&recordSchema=info%3Asrw%2Fschema%2F1%2Fmarcxml-v1.1-light&maximumRecords=10&startRecord=0&recordPacking=XML&availableDBs=jusdb&sortKeys=Submit+query

and bug: query Parameter is at the moment mandatory - even for explain operation which is wrong