Adds the feature maps up, each scaled by its weight.

Mirrors tbp/monty/frameworks/models/salience/strategies/vocus2/vocus2.py::WeightedSum. A weight of zero silences a channel without having to reconfigure the strategy that produced it.

Note the difference from WeightedMean: this one does not renormalise, so a config's weights set the overall scale of the salience map as well as the relative importance of the channels.