Distribute the various external files referenced throughout the tutorials

[BvB93]

A number of .xyz, .psf and other files are currently referenced throughout the tutorials.
Ideally we'd distribute them such that users can more easilly explore the examples.

[BvB93]

Proposed distribution model: add a dedicated module for each tutorial wherefrom one can import a str or pathlib.Path` pointing to the relevant external files.

Examples

For Preparing a Simulation Box:

>>> from tutorials.simulation_box import box_ordered_xyz

>>> print(repr(box_ordered_xyz))
".../box_ordered.xyz"

[BvB93]

Or store them in dictionaries perhaps?

>>> import tutorials

>>> print(tutorials.simulation_box)
{"box_ordered.xyz": ".../box_ordered.xyz"}