Add first cut of tutorial

tutorial/01-single-cell-introduction.ipynb

tutorial/02-single-cell-intermediate.ipynb

tutorial/03a-single-cell-advanced.ipynb

tutorial/03b-networks-introduction.ipynb

tutorial/Acker2008.swc

tutorial/single_cell_allen.swc

tutorial/single_cell_allen_fit.json

tutorial/single_cell_allen_neuron_ref.csv

tutorial/utils.py