Separate layer visibility from feature visibility.
Add vtkFeature::IsVisible() method to combine feature & layer visibilities
Merge pull request #58 from OpenGeoscience/john/extend-visibility

Separate layer visibility from feature visibility.
vtkFeatures now properly delete their vtkLayer.
Properly delete all vtk objects when vtkMap is deleted.

vtkMap never properly deleted any feature or layer.
Merge pull request #62 from robertmaynard/stopVTKLeaks

Properly delete all vtk objects when vtkMap is deleted.
Merge branch 'master' into john/show-area
Prune conus bounds a bit, so that default display has 1 marker  & 1 cluster