DataSet extends Visualizer so that it can be used with ResultCollector.
Classes implementing the interface should create a new instance of
ResultCollector and call setListener(Visualizer) passing itself.
When the ResultCollector.loadExistingFile is called, it will pass
the SampleResults.
In some cases, we may want to return a string that isn't the full
datasource string or something different. For example, we may
want to return just the filename and not the absolutePath of
a JTL file.