Add water fugacity to diffusion creep

[danieldouglas92]

Pull Request Checklist. Please read and check each box with an X. Delete any part not applicable. Ask on the forum if you need help with any step.

Describe what you did in this PR and why you did it.

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.

[bobmyhill]

You will also need to:

• run the indent script after making the changes
• provide a new testcase and
• add a changelog entry.

[naliboff]

An important note: this (common) formulation implies that H2O is a vital chemical component in allowing minerals to deform. Using this formulation, as H2O concentration -> 0, viscosity -> infinity. This is not a reasonable assertion. However, the formulation is empirically justified within a limited range of H2O concentrations. You should probably add a note to say that the H2O concentration compositional field should always have positive values to avoid infinite viscosities.

@bobmyhill - Thanks for that overview and the review. I see two options for enforcing a minimum value:

1. Set a minimum value when passing the water content to the rheology modules (or within the rheology modules), similar to what we do for the pressure in the Drucker Prager rheology module or
2. Modify the reaction terms +/- particle property to enforce a minimum value.

I think option 1 is the most straightforward, but I don't have a strong preference as of now.

@danieldouglas92 - Some testing will be needed to see what the minimum value should be. Ideally, we would converge as closely as possible to the rheology given by only the "dry" values.

[danieldouglas92]

I updated the code and addressed your comments @bobmyhill but at the moment the method John and I talked about required adding an argument to compute_viscosity that I hardcoded to be 1 inside rheology/composite_visco_plastic.cc and diffusion_dislocation.cc, definitely not ideal but thought it would be useful to push anyway to see if you had any thoughts on an approach

[bobmyhill]

@danieldouglas92 did you want to take another look at this, or shall we close it?

[danieldouglas92]

This functionality has been merged in #5678.