c++ implementation and Python bindings of the free foward dynamics (differential action model)

Merged Carlos Mastalli requested to merge cmastall/crocoddyl:topic/cpp-free-fwddyn into cpp_devel

This PR mainly adds the free forward dynamics used for the talos_arm example. Addtionally, it adds a method to share the data with the cost sum. Finally, it adds unit-test for this differential action model.

Edited by Carlos Mastalli

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