Example fix mac os - Githubissues

Hello, Here are a lot of CMake related bugfixes and improvements:

the CMake file to download AdolC has been moved to cmake/
under MacOS, AdolC has been downloaded for each example. This no longer happens.
Removed find_package() calls from example CMakeLists because targets are passed down from the root CMakeLists.txt anyway.
SNOPT interface headers are now installed in include/PSOPT/snopt
User has to specify if he wants SNOPT headers by defining -DWITH_SNOPT_HEADERS=ON.

Best regards

Philipp

PSOPT / psopt