[GeoMechanicsApplication] Implement 'calculator-based' workflow for the Pw continuum elements

[rfaasse]

We will soon have Pw line elements that use LHS and RHS contribution calculators. With that approach, we no longer need to have separate Pw element classes for transient and steady state solutions. The difference is then captured by providing different sets of calculators.

We would like to follow a similar approach for the Pw continuum elements. One of the problems that needs to be addressed is that at present those elements are derived from the U-Pw continuum elements. However, we'd like to derive them directly from Element (to avoid having a dependency on displacement DOFs). Here we can either change their base classes directly (which requires us to carefully inspect which members of the U-Pw elements they rely on) or we can try to make the Pw line element more generic.

The latter approach is preferred, since the Pw line element is quite clean. Therefore, we have decided to make the (transient) Pw line element more generic. The subissues detail the required changes.

When the first subissues have been completed, next subissues will be created to convert the existing continuum Pw elements to the extended TransientPwLineElement.

[loumalouomega]

PLACEHOLDER?