allow formats to be registered with quoted function name

* results in the named function being called rather than an anonymous copy
* mostly an advantage for debugging
basic implementation of new read.neuron.hxskel function
register new read.neuron.hxskel function
tests
doc fix
graph conversion to avoid the undirected-directed conversion for hxskel
improve tests for hxskel reading
avoid use of subset => check() error
factor out code to normalise_swc column order
Fix reading of amira hxskel files by normalising swc column order
STY: comment typo
implement and test is.hxskel
document the fact that is.amiramesh can cope with an array of bytes
implement and test handling of pre-read bytes by is.hxskel
register is.hxskel to identify hxskel neurons
test that read.neuron can recognise an hxskel neuron regardless of extension
test that we _cant_ read hxlineset neurons for now
Merge branch 'feature/hxskel'

* feature/hxskel:
  test that we _cant_ read hxlineset neurons for now
  test that read.neuron can recognise an hxskel neuron regardless of extension
  register is.hxskel to identify hxskel neurons
  implement and test handling of pre-read bytes by is.hxskel
  document the fact that is.amiramesh can cope with an array of bytes
  implement and test is.hxskel
  STY: comment typo
  Fix reading of amira hxskel files by normalising swc column order
  factor out code to normalise_swc column order
  avoid use of subset => check() error
  improve tests for hxskel reading
  graph conversion to avoid the undirected-directed conversion for hxskel
  doc fix
  tests
  register new read.neuron.hxskel function
  basic implementation of new read.neuron.hxskel function
  allow formats to be registered with quoted function name