Require LCG91
- Increases all requirements to the versions available with LCG91.
- The CI builds both with LCG91 and LCG92 on SLC6 and CentOS7.
- Always use CMake namespaces for ROOT.
- Fix built options to only build Core and Tests by default.
- Move all
find_package
calls to main file so that they can be seen/changed in a single location
Fixes ACTS-409.
Edited by Moritz Kiehn