remaining TODOs in Srivastava 1982 example

[slayoo]

• remove compute_drop_volume_total_volume_ratio from example.py
• remove get_pysdm_secondary_products from example.py
• beef up docstrings (e.g., """interprets parameters from Srivastava 1982 in PySDM context""" in settings.py does not convey the actual logic)
• rerun notebooks to match current PySDM and example code
• GPU
• mention rivastava 1982 package in README?
• unit tests for same effect of changing dt or c
• unit test: frag_mass must be smaller than drop size

[abulenok]

related to https://github.com/open-atmos/PySDM-examples/pull/210

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