normalise_swc now can now set defaults for missing cols
default action of normalise_swc changed to 'default'
default value of PointNo changed to integer sequence
document and export normalise_swc
doc cross-refs
remove obsolete ReplaceLabel argument

* never used
* remove the column yourself if you don't want it!
fix bug in normalise_swc using default values

* was still dropping these missing columns
switch seglist2swc to use normalize_swc
set default Label to 2, since this is expected elsewhere

* TODO change this default?
Merge branch 'feature/normalise_swc'

* closes #171
* feature/normalise_swc:
  set default Label to 2, since this is expected elsewhere
  switch seglist2swc to use normalize_swc
  fix bug in normalise_swc using default values
  remove obsolete ReplaceLabel argument
  doc cross-refs
  document and export normalise_swc
  default value of PointNo changed to integer sequence
  default action of normalise_swc changed to 'default'
  normalise_swc now can now set defaults for missing cols
first steps in importing neuroml (XML) files
ensure we identify default namespace
return cells (still as XML objects) rather than names
rename to read.neuroml since this is all it does so far

* fix typo
* also ensure all xml functions are prefixed with XML::
add process_neuroml_cell

* to process the XML tree for a single cell
functions to convert neuroml cell(s) to nat::neuron

* nb this ignores all proximal segments, which may not do what one wants
n(euro)ml->morphml as appropriate

* since we are only proposing to parse the level 1 morphml subset of neuroml
cable information is optional in neuroml
trap neuroml2 files
fix bugs due to earlier renaming