Clamp built-in predicted CoM height to positive vertical velocity

[mickaelbegon]

Summary

• clamp the vertical CoM velocity to its positive part in MINIMIZE_PREDICTED_COM_HEIGHT
• update the penalty test to cover the new behavior explicitly
• document the built-in objective with max(CoM_dot[2], 0) in the README

Validation

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 MPLCONFIGDIR=/private/tmp/mplconfig_bioptim PYTHONPATH=/Users/mickaelbegon/Documents/Synchro_jump/bioptim_pr /Users/mickaelbegon/miniconda3/envs/synchro-jump/bin/python -m pytest /Users/mickaelbegon/Documents/Synchro_jump/bioptim_pr/tests/shard4/test_penalty.py -q -k predicted_com_height

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This change is 

[pariterre]

@mickaelbegon
I am not sure I understand the purpose of this. Why do we assume com cannot be below 0. At if the feet are not at zero? Maybe we could add a "floor_level" value?
Also, I feel using a square objective is much more stable and RAM efficient than clipping to 0 using an "casadi_if_else" statement

[mickaelbegon]

in fact, we don't want the solver to use a negative CoM velocity. The previous form needs to come with a constraint of positive CoMDot... perhaps only add this information to the Readme