Use OperatorGroup for constrain and callback features

[skim0119]

resolve #401 resolve #402

Key update

• Previously, only synchronize features used OperatorGroup. Now, both callback and constraint features use OperatorGroup to manage the operations.
• Finalize methods is not converted since it is not specifically associated to a features (like FreeBC or OneEndFixedBC), but rather associated to a group (like Constraints or Forcing). Hence it does not require more complicated structure to manage the operation.

Minor update

• partially resolve #402: explicit_steppers and related features are now moved to experimental folder.
  • This was because StepType was being refactored from rest of the OperatorType, and type-hinting for explicit stepper was causing more issues.
• OperatorType now specifically means apply_forces, apply_torques, constrain_rates, etc.
  • OperatorType must take time as parameter inputs.
  • OperatorCallableType must take time and current_step as a parameter input.
• StepType now specifically means callable functions used during the timestepping.
• New test: condition to call callback at zero-step
• Pytest-mock is added as part of dev-dependency for easier mock-testing.