add find.soma function similar to find.neuron
teach find.soma to use attached df when possible

* i.e. to use soma position encoded in X,Y,Z columns of attached data.frame
* also remove theshold argument since there is only 1 soma per neuron
Merge branch 'mnqslbs2-find.soma'

* mnqslbs2-find.soma:
  teach find.soma to use attached df when possible
  add find.soma function similar to find.neuron
  closes #164