Verbatim copy of write.neuron
Basic implementation of write.neuron

* note that most of the logic that handles filenames has been handed off to 
  getformatwriter and friends and is any case now rather cleaner.
Fix bug in handling ext=NA - should not be used as query
Full set of tests for rds format including chosen filename / ext
verbatim copy of WriteSWCFile
full implementation of write.neuron.swc + add to registry
test write.neuron.swc - including round trip test
Add verbatim copy of WriteNeuronToAM3D
Add MakeDir and Force arguments to write.neuron

* this way the functionality to check for dirs / overwriting can immediately
  be shared across all neuron formats
Full implementation of write.neuron.hxskel
make sure that getformatwriter asks for fileformats with write=TRUE

* otherwise if a reader is available but a writer is not you get a very 
  strange-looking error message
better error message when no format available to meet specifications
register write.neuron.hxskel
test writing to amira hxskel (including round trip) and unknown format
Verbatim copy of WriteNeuronToAM
full implementation of write.neuron.hxlineset
register write.neuron.hxlineset
test writing in hxlineset format
fileformats(,rval='info') now returns one format per _row_
Some more examples and other minor doc fixes relating to fileformats