a multitude of fixes to test cases

src/pystencils/backend/functions.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/fast_approximation.py

src/pystencils/kernelcreation.py

src/pystencils/spatial_coordinates.py

src/pystencils/types/parsing.py

tests/test_Min_Max.py

tests/test_assignment_collection.py

tests/test_assignment_from_stencil.py

tests/test_astnodes.py

tests/test_conditional_field_access.py

tests/test_custom_backends.py

tests/test_dtype_check.py

tests/test_field.py

tests/test_finite_differences.py

tests/test_floor_ceil_int_optimization.py

tests/test_gpu.py

tests/test_half_precision.py

tests/test_helpful_errors.py