Added Step Back and Restart feature

[jpd-de]

This allows to jump back to the last frame or restart the visualization with a control button.

changed title of step forward button to ">>" and called the Step back button "<<".
Sorted it like "<< Play >>" on a remote control.

[SamTov]

I really like the PR. I do have one critical comment regarding getting the run time as for a static mesh object, this will be 1 which is then wrong for the other meshes. However, overall, really cool job.

If you can fix up the trajectory length (maybe use max instead of min) and make sure the tests are passing, I am happy to merge.

[jpd-de]

I ran the tests locally on my machine and it worked:

platform linux -- Python 3.10.14, pytest-6.2.5, py-1.11.0, pluggy-1.4.0
rootdir: /tikhome/jdrotleff/Packages/ZnVis
plugins: dash-2.16.1
collected 13 items

CI/integration_tests/test_simple_spheres.py . [ 7%]
CI/unit_tests/bounding_box/test_bounding_box.py .. [ 23%]
CI/unit_tests/mesh/test_cylinder.py .. [ 38%]
CI/unit_tests/mesh/test_mesh.py . [ 46%]
CI/unit_tests/mesh/test_sphere.py .. [ 61%]
CI/unit_tests/particle/test_particle.py .. [ 76%]
CI/unit_tests/transformations/test_rotation_matrices.py . [ 84%]
CI/unit_tests/visualizer/test_visualizer.py .. [100%]

======================================================== 13 passed in 13.82s =========================================================

[SamTov]

Happy with the changes.