refactor KernelFunction

src/pystencils/backend/__init__.py

src/pystencils/backend/arrays.py

src/pystencils/backend/ast/__init__.py

src/pystencils/backend/ast/analysis.py

src/pystencils/backend/ast/kernelfunction.py

src/pystencils/backend/constraints.py

src/pystencils/backend/emission.py

src/pystencils/backend/jit/cpu_extension_module.py

src/pystencils/backend/jit/jit.py

src/pystencils/backend/jit/legacy_cpu.py

src/pystencils/backend/kernelcreation/context.py

src/pystencils/backend/kernelfunction.py

src/pystencils/display_utils.py

src/pystencils/kernelcreation.py

src/pystencils/sympyextensions/__init__.py

tests/nbackend/kernelcreation/test_domain_kernels.py

tests/nbackend/test_code_printing.py

tests/nbackend/test_cpujit.py