Refactor source tree layout

.flake8

.gitignore

.gitlab-ci.yml

conftest.py

pyproject.toml

pytest.ini

quicktest.py

setup.py

src/pystencils/__init__.py

src/pystencils/_version.py

src/pystencils/alignedarray.py

src/pystencils/assignment.py

src/pystencils/astnodes.py

src/pystencils/backends/__init__.py

src/pystencils/backends/arm_instruction_sets.py

src/pystencils/backends/cbackend.py

src/pystencils/backends/cuda_backend.py

src/pystencils/backends/dot.py

src/pystencils/backends/json.py

src/pystencils/backends/ppc_instruction_sets.py