add reference for Lee et al 2012 (Drosophila)
no need for triple dots for nat::ngraph

* It’s an exported function
simplify definition of cell
ensure that neuronlist and data.frame have names

* neuronlist had empty (but non-null) names and data.frame had default numeric 
  rownames
messages
first example plots for Sumbul
Merge branch 'master' of github.com:jefferis/nat.examples

* 'master' of github.com:jefferis/nat.examples:
  typo in code