Basic implementation of as.mesh3d.hxsurf
add test for conversion of hxsurf to mesh3d
Add basic implementation of subset.hxsurf
@family hxsurf directives in help
implement as.mesh3d with support for subsetting by region
teach subset.hxsurf to drop unused points

# false by default
as.mesh3d.hxsurf drops unused points by default
Merge branch 'feature/as.mesh3d'

* closes #111
* feature/as.mesh3d:
  as.mesh3d.hxsurf drops unused points by default
  teach subset.hxsurf to drop unused points
  implement as.mesh3d with support for subsetting by region
  @family hxsurf directives in help
  Add basic implementation of subset.hxsurf
  add test for conversion of hxsurf to mesh3d
  Basic implementation of as.mesh3d.hxsurf
ensure as.mesh3d(, drop) is documented
Bump version to 1.5.1

* nb there was no 1.5.0
Bump version for development
Remove brokeen SegLengths field from Cell07PNs

* SegLengths needs to be updated when neurons are
  transformed - this is something that xform should
  probably handle
tidy up and export seglengths function

* including test
Verbatim copy of segmentgraph from nat
functional implementation of segmentgraph

* with test
give seglengths an all argument (default FALSE)
give segmentgraph a weights argument (default TRUE)
remove cross-reference to resample

* keeping these features separate for the moment
Merge branch 'feature/segmentgraph'

* closes #82
* feature/segmentgraph:
  remove cross-reference to resample
  give segmentgraph a weights argument (default TRUE)
  give seglengths an all argument (default FALSE)
  functional implementation of segmentgraph
  Verbatim copy of segmentgraph from nat
Add as.seglist.neuron

* note that when all=TRUE & flatten=FALSE it will return
  a list of seglists
* closes #120