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Bond-orientational order parameters
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``Frame.steinhardt(l)`` returns the local ``q_l`` and the Lechner-Dellago average for every atom.
The neighbour list is the cutoff list.
Degrees 3, 4, 6, 8, and 12 are computed.
``seams steinhardt`` uses the mutual four-nearest list, so its means on the mixed frame are not these means.

The runnable copy is ``docs/tutorials/test_order_parameters.py``.

.. code:: python

    import pydseams as ds

    frame = ds.read("input/traj/exampleTraj.lammpstrj", atom_type=2)
    q3 = frame.steinhardt(3)

``q3["ql"]`` and ``q3["ql_bar"]`` are the per-atom arrays.
On this frame the finite means are about 0.374897 and 0.187542.

The local parameter is



.. math::

    q_l(i) = \sqrt{\frac{4\pi}{2l+1} \sum_{m=-l}^{l} |q_{lm}(i)|^2}

with



.. math::

    q_{lm}(i) = \frac{1}{N_b(i)} \sum_{j \in N(i)} Y_{lm}(\hat r_{ij}).