first sketch of translation context and iteration domain

src/pystencils/nbackend/arrays.py

src/pystencils/nbackend/ast/kernelfunction.py

src/pystencils/nbackend/constraints.py

src/pystencils/nbackend/jit/cpu_extension_module.py

src/pystencils/nbackend/translation/__init__.py

src/pystencils/nbackend/translation/context.py

src/pystencils/nbackend/translation/field_array_pair.py

src/pystencils/nbackend/translation/iteration_domain.py

src/pystencils/nbackend/typed_expressions.py

tests/nbackend/test_basic_printing.py

tests/nbackend/test_cpujit.py

tests/nbackend/test_expressions.py