[WIP] Tectonic-dependent dynamic friction weakening

[Minerallo]

In global models, plastic strain weakening is often unreliable at low resolution due to poor strain localization, which leads to an underestimation of lithospheric weakening. To initiate plate-like behavior, models frequently compensate by reducing the frictional strength to unrealistically low values.

As an alternative, dynamic friction weakening, where the friction angle decreases with increasing strain rate, enables a more physically realistic representation of lithospheric strength.

However, the minimum strength of plate boundaries may vary depending on the tectonic setting (e.g., melt-induced weakening at mid-ocean ridges or fluid-assisted weakening in subduction zones).To address this, I have implemented a new friction weakening model that uses the trace of the strain rate tensor to distinguish between convergent and divergent regions. This allows the model to apply different minimum friction angles based on tectonic context.

Note: I realised that using the trace of the strain rate tensor (equivalent to velocity divergence) may not be the most accurate approach in incompressible models. I plan to investigate using the eigenvalues of the strain rate tensor instead, to more reliably identify compressional and extensional regimes.

Before your first pull request:

• I have read the guidelines in our CONTRIBUTING.md document.

For all pull requests:

• I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• I have tested my new feature locally to ensure it is correct.
• I have created a testcase for the new feature/benchmark in the tests/ directory.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.