Naming: `utils.weighted_sum` computes weighted mean

[vpratz]

@stefanradev93 As this was recently introduced, I wanted to ask whether the naming here was deliberate, or whether we opt for weighted_mean if this is what it computes (this would also follow the language in the corresponding docstring).

The deprecated aggregate() also still shows up in a couple of places (FFF, some scores; search for self.aggregate), so this is something we will want to fix as well.

[stefanradev93]

Good catch! It needs to be renamed and also aggregate needs to be removed.

PS. Technically, we are computing neither a weighted sum, nor a weighted mean; rather, we are computing a mean of weighted elements, since we are not normalizing the weights in each batch (undesirable).

[vpratz]

Fixed by #407.