New feature: function to visually compare neurons

[schlegelp]

Would work with distdots and flag e.g. branches that exist in one but not the other neuron. The main use case I have in mind is to facilitate visual inspection of matches generated by NBLAST. This would hence need to be tightly intertwined with the plotting methods. Might even write this primarily as plotting function.

This could be implemented as a type of NBLAST that instead of a single combined score, returns a score for every node.