Revert previous changes to frontend API

pyproject.toml

src/pystencils/__init__.py

src/pystencils/assignment.py

src/pystencils/backend/kernelcreation/analysis.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelcreation/iteration_space.py

src/pystencils/boundaries/boundaryconditions.py

src/pystencils/boundaries/boundaryhandling.py

src/pystencils/fd/finitedifferences.py

src/pystencils/kernel_decorator.py

src/pystencils/kernelcreation.py

src/pystencils/placeholder_function.py

src/pystencils/rng.py

src/pystencils/simp/__init__.py

src/pystencils/simp/assignment_collection.py

src/pystencils/simp/simplifications.py

src/pystencils/simp/simplificationstrategy.py

src/pystencils/simp/subexpression_insertion.py

src/pystencils/sympyextensions/__init__.py

src/pystencils/sympyextensions/astnodes.py