GeneratorInterface/Core: drop VLA of non-POD type

The patch resolves Clang compile errors regarding usage of VLA with
non-POD types. VLA are not part of C++ standard. It's GNU extension
originating from GCC. Use a standard array on the heap instead as
performance is not a priority in `endJob()`.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>