Add torque-control mode, refactoring in the simulation and mechanics modules

[mhinkkan]

This pull request contains the following changes:

• An unnecessary set_initial_values() method from the simulation module has been removed.
• The time values corresponding to the solution are now saved to all subsystem. This simplifies post-processing the data after the simulation in some cases (if the class uses functions of the simulation time).
• The ExternalRotorSpeed class has been added to the mechanics module. This allows to simulate drive systems in torque-control mode. One example simulation script has also been added.
• The parameters of the TwoMassMechanicalSystem class are now provided through the TwoMassMechanicalSystemPars dataclass.
• The complex exponent is used an internal state for rotating angles in system models. This allows to avoid discontinuities originating from the angle wrapping, which could otherwise cause problems to the solver.
• Notation for the external load torque has been simplified. The speed-dependent load torque term is now parameterized through the speed-dependent friction coefficient (can be constant or callable).