derive velocity predictors

[rfl-urbaniak]

This PR introduces other agents' velocities` as the basis of a new derived predictor.

• velocity.py introduces functions needed to add velocities to forager data and to generate velocity scores.
• derive.py is updated to allow for optional inclusion of velocity-based predictors in the derivation.
• some refactoring and visualization of velocities in animate_foragers.py.
• some downstream refactoring of the existing notebooks, they have been re-run, double-check the content as a sanity check.
• two inferential experiments with velocities are now in velocity_derived_predictors.ipynb.

What is not included in this PR is transformations of filtered foragers data, i.e. no averaging, no selecting the nearest agent, no selecting the fastest agent to copy from etc. These mechanisms will have to be implemented in further steps.

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