scans a directory for files like *.GZ or *.ZIP and returns
        the filename of the first one found (should only be one of
        each file here

def get_file_named(self, fldr, xtn):
        """
        scans a directory for files like *.GZ or *.ZIP and returns
        the filename of the first one found (should only be one of
        each file here
        """
        res = []       # list of Sample objects
        for root, _, files in os.walk(fldr):
            for basename in files:
                if fnmatch.fnmatch(basename, xtn):
                    filename = os.path.join(root, basename)
                    res.append(filename)

        if len(res) > 0:   
            return res[0]
        else:
            return None