Properly exclude meta examples from build

[karlnapf]

Certain meta examples are excluded from the build, if they have optional dependencies that are not met (e.g. GPL, NLOPT, etc). This is done here: https://github.com/shogun-toolbox/shogun/blob/develop/cmake/FindMetaExamples.cmake#L18

Currently, we do not even generate the code listings of excluded examples. This is bad since when generating documentation, sphinx will complain about missing code listings (an example page for the cookbook exists, but no example).

To avoid the need to have all dependencies installed, example listings should always be generated, but rather be excluded from the tests.

Good task for someone to learn about cmake, not too difficult but not trivial either

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.