Basic implementation of read.neuron.fiji

* pretty much straight from AnalysisSuite
Don't set width to dummy value

* use seglist2swc to process partial input data
Document and export read.neuron.fiji
Basic tests for reading fiji neurons
rename internal function to read.fijixml
fix reading of files with multiple neurons

* should return a neuronlist
Support for SWC Label

* and cross-check our import with Fiji SWC export
is.fijixml to recognise file
rename to is.fijitraces and document
register file format (and use read.neuron)
Document support for Fiji traces in read.neurons
Merge branch 'feature/read.neuron.fiji'

* closes https://github.com/jefferis/nat/issues/178
* feature/read.neuron.fiji:
  Document support for Fiji traces in read.neurons
  register file format (and use read.neuron)
  rename to is.fijitraces and document
  is.fijixml to recognise file
  Support for SWC Label
  fix reading of files with multiple neurons
  rename internal function to read.fijixml
  Basic tests for reading fiji neurons
  Document and export read.neuron.fiji
  Don't set width to dummy value
  Basic implementation of read.neuron.fiji
Fix is.nrrd for raw input

* fixes https://github.com/jefferis/nat/issues/186
Add is.swc() to test files for SWC format

* includes tests
Check if file content looks like SWC as last resort
Minor doc fixes/improvements