Plotting notebook: added plt.figure() calls

in unit testing without display plots, the same figure would be used
over and over again. This is problematic when mixing 3D and 2D axes
7 jobs for master in 5 minutes and 24 seconds (queued for 7 minutes and 28 seconds)
Name Stage Failure
failed
tests-and-coverage Test
3.07s call     pystencils_tests/test_datahandling_parallel.py::test_kernel
2.78s call pystencils_tests/test_buffer.py::test_all_cell_values
2.74s call pystencils_tests/test_cudagpu.py::test_periodicity
2.63s call doc/notebooks/03_tutorial_datahandling.ipynb::doc/notebooks/03_tutorial_datahandling.ipynb
======== 5 failed, 195 passed, 3 skipped, 359 warnings in 89.75 seconds ========
Uploading artifacts...
coverage_report: found 75 matching files
Uploading artifacts to coordinator... ok
id=293610 responseStatus=201 Created token=X1Lu6v39
ERROR: Job failed: exit code 1