Fixed offline generation of rdf for chained transforms

WorksheetR2RMLJenaModelParser needed to be updated to use
WorksheetCommandHistoryReader.executeListOfCommands().  Otherwise, it
tried to verify every command had a valid hNode source before applying
any column transformations, which caused transforms that operated on
anything that wasn't in the original source to be thrown out.

Some more clean up of unused imports and some non-static accesses.