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