Add Compositional Plastic Weakening Factors [WIP]

[danieldouglas92]

A project that @Geoniette, @Minerallo, and @Djneu talked about at the Hackathon. This pull request enables the user to apply weakening factors to the plastic parameters (cohesion and friction angle) based on a compositional field. The current case that is presented in this PR is specifically for the porosity compositional field. This could be extended to more properly handle how fluid weakens the plastic parameters through the fluid pressure, but this implementation currently approximates how the presence of fluid reduces the yield strength.

The user can specify a minimum cohesion and friction angle, and how much porosity is required for the model to reach these minimum values. The cohesion and friction angle are linearly reduced for porosity values between 0 and the upper limit on the porosity. I've created some simple tests which show that the cohesion and the friction angle are actually reduced where there is porosity, and this also works in more complicated setups (see images below).

The modeled cohesion (Top), friction angle (middle), and yield stress (bottom) for a subduction model. Two distinct regions of reduced cohesion and friction angle correlate to where fluids are being released from the subducting plate. The yield stress is only reduced closer to the model surface because of the `Maximum yield stress` parameter.

The implementation largely follows what was done for the compositional viscosity prefactors implemented in #5678. I need to go back and double check the way that this is being applied though. If this feature is the only plastic weakening method used I think it is fine, but I am not confident that if this is used with strain weakening for example that both mechanisms are being applied correctly.