Filter step in the Transform tab

[herminiogg]

It could be useful to have the possibility to filter the XML files based on a given expression (very useful for OAI-PMH). It could be for example an XPath expression that depending on its evaluation (true or false) filters or not the given XML file. Alternatively, it can be a similar system that given an XPath expression lets to select automatically the files that accomplish the filtering expression in the upload or ingest tab.