as.neuronlist df arg defaults to NULL

* since this is already true for neuronlist
empty names for neuronlist = null names

* sometimes you get empty names from e.g. lapply operations
add basic test for as.neuronlist
check that we can cope with list with empty or absent names
Merge branch 'fix/as.neuronlist'

* fix/as.neuronlist:
  check that we can cope with list with empty or absent names
  add basic test for as.neuronlist
  empty names for neuronlist = null names
  as.neuronlist df arg defaults to NULL
* closes #163