hemibrain_read_neurons should reroot neurons before splitting them

[jefferis]

@alexanderbates hemibrain_read_neurons first splits the neuron and then reroots on the soma, which means that neurons such as the one below do not benefit from the corrected soma information.

> n912601268.hb = hemibrainr::hemibrain_read_neurons(912601268)
Warning message:
In hemibrain_flow_centrality.neuronlist(neurons, ...) :
  1 neurons have no soma tagged, split could be inaccurate for: 912601268

(replaces natverse/neuprintr#134)