Rename `ballistic` parameter to `mode`

[bqth29]

Currently, to specify whether to use Ballistic SB (bSB) or Discrete SB (dSB), the ballistic parameter must be respectively set to True or False.

However, it can seem confusing to have a ballistic parameter but not a discrete parameter. Besides, new SB modes could emerge in litterature. Thus, it could be relevant to replace the boolean ballistic parameter by a string mode parameter that would have to possible values for now: "ballistic" and "discrete".

While updating this, it could be interesting to refactor the SimulatedBifurcationEngine enum by removing the notion of heating because it is independent from the activation function of each engine. The heat would just be a parameter of the symplectic integrator.

[bqth29]

engine, version or activation could also be good candidates, maybe we should vote