Improve repair by using pool of dendrites (as for axons)

[arnaudon]

This change is beyond the scope of #65 and will be treated in a future PR.
The rational is similar to synthesis, where all dendrites of all morphologies of same mtypes are used to synthesize a morphology, to avoid biases from specific features of morphologies. With this improvement, the repaired branches would be more varied, and not be almost a copy of other branches of the current morphology.