add test cases for pragmas and omp. Remove pragma adding from public interface.

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/emission.py

src/pystencils/backend/kernelcreation/cpu_optimization.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/literals.py

src/pystencils/backend/transformations/__init__.py

src/pystencils/backend/transformations/add_pragmas.py

src/pystencils/config.py

src/pystencils/kernelcreation.py

tests/nbackend/kernelcreation/test_openmp.py

tests/nbackend/test_ast.py

tests/nbackend/transformations/test_add_pragmas.py