Separate layer visibility from feature visibility.
Merge pull request #59 from robertmaynard/missing_pi_define

By default visual studio doesn't provide the M_PI define
Merge pull request #61 from robertmaynard/valgrind_detected_leaks

Valgrind detected leaks
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.
Pass and Return by const where it makes sense.
Merge branch 'stopVTKLeaks'
Merge branch 'const_char_pointers'