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