Use a single reference value in sum tasks

The problem of sum tasks is that having an array of livetime_reference_values meant that different fit results/detector data could be normalized to a different reference value.

As Normalization task, I didn't have to make any changes since the multiple reference values comes from the fact that it normalizes a single fit result (data_uri) with respect to multiple references (in the single detector case: livetime and I0). So we have a reference value per reference but we cannot have a different reference value per fit result since there is only one data_uri as input.

We may need to create a new task to normalize multiple fit results (after the MultiScanFit) ?