Option to summarise attributes when merging multiple edges or contracting nodes

[luukvdmeer]

Is your feature request related to a problem? Please describe.
When simplifying a network, sets of edges that connect the same pair of nodes (i.e. multiple edges) get reduced to one. Currently that is done by simply keeping the first edge of such a set, and removing the others. However, consider a case where you have several edges connecting node A and B, and each edge is describing a flow of a certain transport mode (this example I got from ITSLeeds/od#20). Then, when you want to merge all these edges, you want the resulting edge to have the sum of all flows as flow value, and not just the flow of the first edge. Probably there are more cases where it makes sense to combine the attributes of the original edges in some way (sum, mean, mode, ...). It would be nice to be able to specify this on a per-attribute basis.

The same could be applied to node contraction: when several nodes get contracted into a single one, you may want to specify how you want to combine the node attributes.

[luukvdmeer]

This is implemented now. Examples can be seen in the morpher vignette (contraction and simplification) and in the cleaning vignette (contraction and simplification)