Extend symbolic language support

.gitlab-ci.yml

conftest.py

docs/source/api/index.rst

docs/source/api/symbolic_language/astnodes.rst

docs/source/api/symbolic_language/field.rst

docs/source/api/symbolic_language/index.rst

docs/source/api/symbolic_language/sympyextensions.rst

docs/source/api/sympyextensions/index.rst

src/pystencils/backend/ast/expressions.py

src/pystencils/backend/ast/util.py

src/pystencils/backend/functions.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/kernelcreation/typification.py

src/pystencils/backend/platforms/generic_cpu.py

src/pystencils/backend/platforms/generic_gpu.py

src/pystencils/backend/platforms/platform.py

...pystencils/backend/transformations/eliminate_constants.py

src/pystencils/backend/transformations/select_functions.py

src/pystencils/config.py

src/pystencils/kernel_wrapper.py