CMake - Add one CMakeLists per package

[janetournois]

Summary of Changes

To be able to test a full package (tests, examples, and if they exist benchmark and demo) at once, for example using ctest (or simply compiling in visual studio), I suggest to add one CMakeLists per package, at the root of the package.

This PR introduces two such CMakeLists, which I think are very handy.

[janetournois]

@lrineau for "benchmark" I suggest 808fa60

[sloriot]

Instead of manually creating cmake scripts, can't we create custom targets in case we have CGAL_ENABLE_TESTING=ON?

[janetournois]

I don't like much that option, because it forces to use the top-level CMakeLists, while the solution I suggested is "package-centered"