Merge branch 'oldsympy' into 'master'

.gitlab-ci.yml

conftest.py

doc/notebooks/01_tutorial_getting_started.ipynb

doc/notebooks/demo_plotting_and_animation.ipynb

doc/notebooks/demo_wave_equation.ipynb

pystencils/astnodes.py

pystencils/backends/cbackend.py

pystencils/cpu/cpujit.py

pystencils/fd/derivative.py

pystencils/include/cuda_complex.hpp

pystencils/kernelcreation.py

pystencils_tests/test_boundary.py

pystencils_tests/test_buffer_gpu.py

pystencils_tests/test_complex_numbers.py

pystencils_tests/test_cuda_known_functions.py

pystencils_tests/test_custom_backends.py

pystencils_tests/test_datahandling.py

pystencils_tests/test_field.py

pystencils_tests/test_interpolation.py

pystencils_tests/test_loop_cutting.py