Interface to combine graphs (merging attributes) with custom mapping

The preferred way is slicing of graphs, but merging should definitely be possible and well supported.

a first step (internally?) would be to convert both graphs to the same type. Then the mapping is all about finding the correct subgraph isomorphism.
a first simplification for the subgraph isomorphism should be, that one graph is structurally a strict subgraph of the other.
combining 2 graphs that don't have a strict subgraph relation:
- merge vertices/edges where an appropriate mapping exists. otherwise, copy elements directly and simply add them.
- this might not be possible with the existing boost::graph_copy functionality, but might require iterating over the vertices of a graph and do a lookup in the mapping
- would be easier if the mapping was done as a boost::bimap....

ComputationalRadiationPhysics / DoDo