Merge branch 'Extend_testsuite' into 'master'

pystencils/backends/cbackend.py

pystencils/backends/cuda_backend.py

pystencils/datahandling/datahandling_interface.py

pystencils/datahandling/serial_datahandling.py

pystencils/fd/derivative.py

pystencils/simp/__init__.py

pystencils/simp/assignment_collection.py

pystencils/simp/simplifications.py

pystencils/stencil.py

pystencils/sympyextensions.py

pystencils/transformations.py

pystencils/utils.py

pystencils_tests/test_assignment_collection.py

pystencils_tests/test_astnodes.py

pystencils_tests/test_data/datahandling_load_test.npz

pystencils_tests/test_data/datahandling_save_test.npz

pystencils_tests/test_datahandling.py

pystencils_tests/test_datahandling_parallel.py

pystencils_tests/test_fast_approximation.py

pystencils_tests/test_fd_derivative.py