Initial implementation of nblast_allbyall
teach sub_dist_mat to cope with missing neuron_names

* just use rownames of scoremat
* this allows nhclust to be called with a scoremat and no names
simplify example of all by all nblast + clustering

* not clear that nblast_allbyall is doing much over nblast in this example.
Merge branch 'feature/nblast_allbyall'

* feature/nblast_allbyall:
  simplify example of all by all nblast + clustering
  teach sub_dist_mat to cope with missing neuron_names
  Initial implementation of nblast_allbyall