main-coverity-ITkPixelByteStreamCnv

Trying to address one coverity defect (excessive stack use) by making the (very large) look-up table constexpr. Along the way, discovered (and fixed) a bug in the return of create_lut_encode_ and introduced unit tests to ensure the new code is functional. Also fixed one cppcheck defect (should pass large arguments by const reference). tagging @okovanda and @sroygara as original authors.