Nodestats test_nodestats_image raises runtime warning from calc_angles()

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Running tests I noticed the following being returned...

tests/tracing/test_nodestats.py::test_nodestats_image[catenane]
tests/tracing/test_nodestats.py::test_nodestats_image[catenane]
tests/tracing/test_nodestats.py::test_nodestats_image[replication_intermediate, not pairing odd branches]
tests/tracing/test_nodestats.py::test_nodestats_image[replication_intermediate, pairing odd branches]
  /home/neil/work/git/hub/AFM-SPM/TopoStats/topostats/tracing/nodestats.py:1110: RuntimeWarning: invalid value encountered in arccos
    return abs(np.arccos(cos_angles) / np.pi * 180)  # angles in degrees

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

Relates to the calc_angles() static method (for which we don't currently have tests).

We should fix this, perhaps via a a try: ... except: depending on the nature of the problem, prior to #800 being completed via #932.

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Closed by #956