Make the 'Average' operator the same as a weighted sum with equal weights

When calculating the indices, the user can select between operators 'Average', 'Sum' and 'Product', which determine how to combine indices, themes and indices, to obtain the Integrated Risk Index. The 'Average' operator was not working properly, because it performed a weighted sum and then it also averaged the result. This PR fixes such behavior, making the operator perform a equally-weighted sum, i.e. the sum divided by the number of combined items.