docs: def copy_abiext(self, inext, outext):
          """Copy the Abinit file with extension inext to a new file withw extension outext"""
          infile = self.has_abiext(inext)
          if not infile:
              raise RuntimeError('no file with extension %s in %s' % (inext, self))
  
          for i in range(len(infile) - 1, -1, -1):
              if infile[i] == '_':
                  break
          else:
              raise RuntimeError('Extension %s could not be detected in file %s' % (inext, infile))
  
          outfile = infile[:i] + '_' + outext
          shutil.copy(infile, outfile)
          return 0