Refactoring velocity predictors

[PalkaPuri]

Updated formulation of Vicsek predictors to remove vector averaging of velocity magnitude. The velocity magnitude is now sampled from a gaussian centered at the velocity of previous time step. Previously used _generic_velocity_predictor with a mechanism-specific transformation of interaction partner velocities did not allow for this change in the vicsek predictor. So I have now removed it, and I directly wrote two generate functions for pairwiseCopying and vicsek.

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[rfl-urbaniak]

We updated staging-collab-2, please make sure you pull origin, resolve conflicts and pass the tests.

[PalkaPuri]

I updated velocity_predictors.ipynb to test both pairwiseCopying and vicsek predictors, removed vicsek_predictor.ipynb. Also pruned the notebook to only use the non-underscored (exposed) generate functions. Found a bug in animate_predictors that did not allow plotting positions of a selected set of foragers. Fixed that in this PR as well.

[PalkaPuri]

I deleted compute_velocity.ipynb which tests _add_velocity function since (1) we want to avoid importing underscored functions (2) _add_velocity is implicitly tested when generate_pairwiseCopying_predictor and generate_vicsek_predictor are called in velocity_predictors.ipynb