some more STIR CMake variable changes
some more STIR CMake variable changes

- added STIR_DISABLE_LLN_MATRIX STIR_ENABLE_EXPERIMENTAL
- rename STIR_DISABLE_CERN_ROOT_SUPPORT to STIR_DISABLE_CERN_ROOT to be consistent with recent STIR naming

CCPPETMR/SIRF-SuperBuild,CCPPETMR/SIRF-SuperBuild