Update deprecated loadData usage before diffpy 4.0 removal

[AndrewSazonov]

easyscience/diffraction-lib/src/easydiffraction/datablocks/experiment/item/total_pd.py:74: DeprecationWarning: 'diffpy.utils.parsers.loaddata.loadData' is deprecated and will be removed in version 4.0.0. Please use 'diffpy.utils.parsers.load_data' instead.
data = loadData(data_path)

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

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