[GeoMechanicsApplication] Refactor processing of DOFs by elements and conditions

[avdg81]

As a developer, I would like to define the order of degrees of freedom (in elements and conditions) just once, instead of repeating it over and over again. This will improve the maintenance of the corresponding code. Furthermore, it will become much easier to change the order if needed. In fact, when this issue has been addressed, we would like to change the DOF order of U-Pw same order elements to match the order adopted by U-Pw diff order elements. That is, however, the scope of another issue.