Refactor source tree layout

.flake8

.gitignore

.gitlab-ci.yml

conftest.py

pyproject.toml

pytest.ini

quicktest.py

src/lbmpy/__init__.py

src/lbmpy/_version.py

src/lbmpy/advanced_streaming/__init__.py

src/lbmpy/advanced_streaming/communication.py

src/lbmpy/advanced_streaming/indexing.py

src/lbmpy/advanced_streaming/utility.py

src/lbmpy/analytical_solutions.py

src/lbmpy/boundaries/__init__.py

src/lbmpy/boundaries/boundaries_in_kernel.py

src/lbmpy/boundaries/boundaryconditions.py

src/lbmpy/boundaries/boundaryhandling.py

src/lbmpy/chapman_enskog/__init__.py

src/lbmpy/chapman_enskog/chapman_enskog.py