Fix test that included a check for atime of a file read twice in succession

* noticed by Brian Ripley on Solaris
* MacOSX does not update the atime for this access
* other OSes may do the same or round time to nearest second
  (which would only result in rare failures)
Remove warnings by supplying vertex data for as.neuron.ngraph tests

* real neurons always have vertex data, but for convenience tests omitted this
* reveals a bug in as.neuron.ngraph
Fix handling of as.neuron.ngraph when vertexData does not have PointNo field

* test in previous commit checks for this behaviour
* add additional test for behaviour when vertexData has too many rows
  (and no PointNo field)
Bump version to 1.4.6