Fix: Expression clones should keep their data type

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/ast/structural.py

src/pystencils/backend/extensions/cpp.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/types/parsing.py

tests/nbackend/test_ast.py