Find the label for the output files 
         for this calculation

def _get_label(self):
        '''Find the label for the output files 
         for this calculation
        '''
        if self._label is None:
            foundfiles = False
            for f in self._files:
                if ".files" in f: 
                    foundfiles = True
                    self._label = f.split(".")[0]
                    with open(self._label + '.files', 'r') as fp:
                        line = fp.readline().split()[0]
                        if line != self._label + ".in":
                           fp.close()
                           raise Exception('first line must be label.in')
                        line = fp.readline().split()[0]
                        if line != self._label + ".txt":
                           fp.close()
                           raise Exception('second line must be label.txt')
                        line = fp.readline().split()[0]
                        if line != self._label + "i":
                           fp.close()
                           raise Exception('third line must be labeli')
                        line = fp.readline().split()[0]
                        if line != self._label + "o":
                           fp.close()
                           raise Exception('fourth line must be labelo')
                        fp.close()
            if foundfiles:
                return self._label
            else:
                raise Exception('label.files not found')
                                
#ASE format
#        (self.prefix + '.in') # input
#        (self.prefix + '.txt')# output
#        (self.prefix + 'i')   # input
#        (self.prefix + 'o')   # output
                
        else:
            return self._label