GSF Component Merging : use std::vector for the triangular to (i,j) mapping

The arrays we were using to do the mapping from the triangular array index of the distance matrix to the (i,j) of the component pair are not used in a way that we need to make them aligned

For the float ones is another story and there is an ongoing discussion on possible missed optimisations.

Also added a few comments on what the code is doing.

Mentioning @amorley