Separate layer visibility from feature visibility.
Make sure to use double only when call std::pow.
Merge branch 'unambiguous_pow'
By default visual studio doesn't provide the M_PI define, so work around that.

It is far to complicated to make sure everything gets included in the right
order for that to happen for a header only class. Instead we provide our own
M_PI define if it isn't already defined.
Merge branch 'missing_pi_define'

Conflicts:
	vtkMercator.h
Properly allocate room for the null terminator.

Also properly delete the StorageDirectory path.
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.
Pass and Return by const where it makes sense.