def create(label)
      create_atomic(label.to_s) or create_composite(label.to_s) or raise MeasurementError.new("Unit '#{label}' not found")
    end
def create(label)
      create_atomic(label.to_s) or create_composite(label.to_s) or raise MeasurementError.new("Unit '#{label}' not found")
    end
Returns the unit with the given label. Creates a new unit if necessary.

 Raises MeasurementError if the unit could not be created.