text: Title: Implement a node multiselect feature for the d3 web frontend
  Body: **The problem:**
  A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
  Currently the d3 web frontend only offers a very simple search that highlights a given subset of nodes, based on a single search term. Sometimes it might be useful to only select/highlight a specific subset of nodes that a not based on simple string search criteria. This feature would increase usability, since it would facilitate more explorative approaches and visualization.
  
  **A solution:**
  A user should be able to put together a specific subset of highlighted nodes (optically with the same result as performing a simple node string search in the search box) by simply hovering over a given node, pressing a given key (e.g. space) and then adding/removing the hovered node to a current node highlight selection. The currently implemented search should be compatible with this multiselect feature, i.e. If I search for a given term, I still can add more nodes the the highlighted selection by simply hovering and selecting/deselecting them by pressing a given key.
  
  All highlight logic currently holding for the search should be valid also the the hover/multiselect feature, e.g. if I highlight two connected nodes, the connecting edge should also be highlighted).
  