clean up some APIs with respect to pystencils master

src/pystencils/__init__.py

src/pystencils/fd/derivation.py

src/pystencils/fd/derivative.py

src/pystencils/fd/finitedifferences.py

src/pystencils/field.py

src/pystencils/kernel_decorator.py

src/pystencils/simp/simplifications.py

src/pystencils/simp/subexpression_insertion.py

src/pystencils/sympyextensions/__init__.py