Add GroupBy mean result as a new column in pandas

I have a dataframe that gives upper and lower values of each indicator as follows

    df = pd.DataFrame(
           {'indicator': ['indicator 1', 'indicator 1', 'indicator 2', 'indicator 2'],
            'year':[2014,2014,2015,2015],
            'value type': ['upper', 'lower', 'upper', 'lower'],
            'value':[12.3, 10.2, 15.4, 13.2]
           },
           index=[1,2,3,4])

I want to remove the upper and lower values and replace that with the mean of two values.

How can I do that?