Remove 4 DoF (leave only 6 DoF) model

[jguarato]

The objective of this pull request is to standardize the construction of elements and the assembly of the rotor exclusively on the 6-degree-of-freedom (DoF) model. To achieve this, several measures were implemented to ensure that changes to the code did not introduce any breaking issues. These measures include:

1. Removal of 4 DoF Models: Eliminated the 4 DoF models from all element classes, including ShaftElements, DiskElements, BearingElements, and PointMass.
2. Simplification of Conditional Statements: Removed conditional logic specific to the 6 DoF model to streamline the code.
3. Documentation Updates: Corrected and updated the docstring examples to reflect the changes.
4. Unit Test Adjustments: Fixed unit test results to align with the updated 6 DoF implementation.
5. Adaptation of run_ucs Method: Made necessary adjustments to ensure compatibility with synchronous=True in the run_ucs method.
6. Stochastic Module Compatibility: Applied various fixes to adapt the ROSS stochastic module to the updated model.
7. Integration and Corrections for CouplingElement: Merged and resolved issues in the CouplingElement functionality.
8. Tutorial and User Guide Updates: Revised tutorials and examples in the user guide to reflect the new 6 DoF implementation.