Convert a multi-dim to a pandas dataframe.

def to_dataframe(self, columns=None):
    """Convert a multi-dim to a pandas dataframe."""
    if not isinstance(self.measured_value, DimensionedMeasuredValue):
      raise TypeError(
        'Only a dimensioned measurement can be converted to a DataFrame')

    if columns is None:
      columns = [d.name for d in self.dimensions]
      columns += [self.units.name if self.units else 'value']

    dataframe = self.measured_value.to_dataframe(columns)

    return dataframe