Speed up CI

Closes #64 (closed)

• Run clang-scan-build only on scheduled pipelines
• Use ccache for all (regular) C++ jobs
• Only run C++/Python jobs if necessary (checking if code files are changed)
• Disable running branch pipelines on non-main branches (can still be triggered via web UI)

TODO:

• Test Python is only run when necessary
• Test C++ is only run when necessary
• Disable ccache for scheduled jobs
• Check coverage reporting in GitLab MR UI