Fix colouring of segments by plot3d.dotprops

* fixes #123
always drop unused vertices in as.mesh3d.hxsurf

* even if we do not drop any regions
* closes #124
teach subset.hxsurf to accept regexes
Give subset.hxsurf an rval argument to allow it to return names

* will use this in plot3d.hxsurf
Teach plot3d.hxsurf to accept regexes for materials

* making use of subset.hxsurf
Merge branch 'feature/hxsurf-regexes'

* closes #122
* feature/hxsurf-regexes:
  Teach plot3d.hxsurf to accept regexes for materials
  Give subset.hxsurf an rval argument to allow it to return names
  teach subset.hxsurf to accept regexes
Add failing test when write.neurons had subdir=NULL
fix bug in write.neurons when subdir=NULL

* fixes #125
additional tests when using a property of neuron as subdir
Bump version to 1.5.2
Add zenodo doi badge
Change Zenodo badge to SVG version
Merge pull request #126 from ajdm/zenodobadge

Change Zenodo badge to SVG version
Fix read.hxsurf to order vertices st surface normals always point outwards
Merge pull request #127 from ajdm/fix/surfacenormals

Fix read.hxsurf to order vertices such that surface normals always point outwards
Update nat citation with Zenodo DOI
Add basic implementation of pointsinside based on Rvcg

* Rvcg::vcgClost has a signed distance function but the result
  only seems to make sense for whole brain surfaces
* but not neuropil surfaces
* assume this has to do with how they are closed
Update to match up with fixed surface normals
Add test for pointsinside
minor doc fix - prune.dotprops has moved to nat