Better docs for cell, charges, positions

[PicoCentauri]

The shapes and the contents of these parameters are not clear in the current version. Now we (hopefully) use the same docstring for all of these.

I also used lower cases for torch.tensor in the docstring to be consistent with other types like list, dict, etc.

For a review it is enough to check if these lines are okay

        :param charges: torch.Tensor of shape ``(n_channels, len(positions))``
             containaing the atomic (pseudo-)charges. ``n_channels`` is the number of
             charge channels the potential should be calculated. For a standard potential
             ``n_channels = 1``. If more than one "channel" is provided multiple
             potentials for the same position are computed depending on the charges and
             the potentials.
         :param cell: torch.tensor of shape ``(3, 3)``, where ``cell[i]`` is the i-th basis
             vector of the unit cell
         :param positions: torch.tensor of shape ``(N, 3)`` containing the Cartesian
             coordinates of the ``N`` particles within the supercell.
         :param neighbor_indices: torch.tensor with the ``i,j`` indices of neighbors for

and a slightly shorter version for the charges in some modules.

    :param charges: torch.tensor of shape ``(len(positions, 1))`` containing the atomic
         (pseudo-)charges

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?

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📚 Documentation preview 📚: https://torch-pme--160.org.readthedocs.build/en/160/

[E-Rum]

Should it be torch.tensor instead of torch.Tensor?

[PicoCentauri]

Should be done.