Percolation tests are slow

[stefsmeets]

I noticed tests are much slower after #178. It seems like the percolation test takes the full dataset, and therefore takes over 40s to complete. I think we should use a reduced dataset for this.

======================== slowest 10 durations ========================
39.53s call     tests/integration/transitions_test.py::test_transitions_find_best_perc_path
5.73s setup    tests/integration/shape_test.py::test_shape
5.59s setup    tests/integration/plot_test.py::test_shape[png]
2.78s setup    tests/integration/rdf_test.py::test_rdf
2.23s setup    tests/integration/plot_test.py::test_rdf[png]
1.76s call     tests/integration/plot_test.py::test_jumps_3d_animation[png]
1.26s call     tests/integration/plot_test.py::test_jumps_3d[png]
1.12s call     tests/integration/trajectory_test.py::test_volume_to_structure_cluster

[stefsmeets]

On a side note, this also means the algorithm itself is quite slow. Is there anything we can do about that?