more simplifications, cleanup, and notes

src/pystencils/nbackend/arrays.py

src/pystencils/nbackend/ast/kernelfunction.py

src/pystencils/nbackend/kernelcreation/context.py

src/pystencils/nbackend/kernelcreation/defaults.py

src/pystencils/nbackend/kernelcreation/domain_kernels.py

src/pystencils/nbackend/kernelcreation/freeze.py