Updated and organized TODOs. Fixed minor TODOS.

pystencils/assignment.py

pystencils/astnodes.py

pystencils/backends/cbackend.py

pystencils/backends/cuda_backend.py

pystencils/bit_masks.py

pystencils/cache.py

pystencils/config.py

pystencils/cpu/kernelcreation.py

pystencils/cpu/vectorization.py

pystencils/fast_approximation.py

pystencils/field.py

pystencils/functions.py

pystencils/integer_functions.py

pystencils/kernel_contrains_check.py

pystencils/kernelcreation.py

pystencils/simp/simplifications.py

pystencils_tests/test_types.py