Use StrictUndefined and add CustomFunctionCall

src/pystencils_autodiff/_file_io.py

src/pystencils_autodiff/backends/astnodes.py

src/pystencils_autodiff/backends/module.tmpl.hpp

src/pystencils_autodiff/field_tensor_conversion.py

src/pystencils_autodiff/framework_integration/astnodes.py

src/pystencils_autodiff/framework_integration/printer.py

...encils_autodiff/framework_integration/texture_astnodes.py

src/pystencils_autodiff/framework_integration/types.py

src/pystencils_autodiff/graph_datahandling.py

src/pystencils_autodiff/tensorflow_jit.py

tests/test_dynamic_function.py