fix test for ngraph attributes

* assume the original test for class was a typo since it is null
Add option to set vertex attributes (and test this)
as.neuron.ngraph should always pass on ...

* wasn't doing this for seglist method
* closes #182
XYZ now vertex attributes of ngraph

* rahter than graph attributes
* closes https://github.com/jefferis/nat/issues/180
Define directionality of ngraph edges
set diam information in ngraph object

* and use this in generic functions
* TODO use this in other functions and test
* closes https://github.com/jefferis/nat/issues/181
* see also https://github.com/jefferis/nat/issues/180
normalise_swc normalises rownames (by setting to NULL)

* fixes https://github.com/jefferis/nat/issues/183
* this should avoid some ndigest issues, but could
  also cause trouble for old neurons R<2.4.0 when the row.names were character vectors
Update baselines given small change in normalise_swc
Put coords into ngraph in read.neuron.hxlineset

* this is for style really, but should be identical
document ngraph(diag)
option to set diameter in xyz arg
Merge branch 'feature/ngraph-atts'

* feature/ngraph-atts:
  option to set diameter in xyz arg
  document ngraph(diag)
  Put coords into ngraph in read.neuron.hxlineset
  Update baselines given small change in normalise_swc
  normalise_swc normalises rownames (by setting to NULL)
  set diam information in ngraph object
  Define directionality of ngraph edges
  XYZ now vertex attributes of ngraph
  as.neuron.ngraph should always pass on ...
  Add option to set vertex attributes (and test this)
  fix test for ngraph attributes