Add some combining operators

This PR improves the operators used to calculate the indices and their sub components. In the previous situation, we had 3 operators: sum (weighted), average and multiplication.
Chris asked to provide also a couple of more "naive" operators, i.e., a basic (not weighted) sum (which provides unbalanced results, but can be useful in some contexts), and a basic (not weighted) multiplication (same as above).
The combination types are now better factorized and defined only once.