Function from_multi_xml 

pub fn from_multi_xml<'a>(
    filenames: &Vec<&Path>,
    outputfile: Option<&Path>,
    config: &XmlConversionConfig,
    store: &'a mut AnnotationStore,
) -> Result<(), String>

Translate an XML file to STAM, given a particular configuration. This translates multiple XML files to a single output file.