Add function to prepend pragmas to loops

Add a function to prepend #pragma directives to domain loops according to their nesting depth. Useful for compiler-specific annotations and interactions, especially with less well-known niche compilers (like Fujitsu's ARM-compiler).

pystencils/cpu/__init__.py

 from pystencils.cpu.cpujit import make_python_function
-from pystencils.cpu.kernelcreation import add_openmp, create_indexed_kernel, create_kernel
+from pystencils.cpu.kernelcreation import add_openmp, create_indexed_kernel, create_kernel, add_pragmas
 
-__all__ = ['create_kernel', 'create_indexed_kernel', 'add_openmp', 'make_python_function']
+__all__ = ['create_kernel', 'create_indexed_kernel', 'add_openmp', 'add_pragmas', 'make_python_function']