[WIP] Viscoplastic lookup table

[ryanstoner1]

A modification to the visco_plastic material model to optionally set viscosities with lookup tables. These tables are used in place of temperature or pressure conditions by reading a column for dominant phases. What the dominant phase is is checked against a user-provided list of phases of interest.

Update: The goal is to be able to replace the existing analytical phase functions with phase functions read from a lookup table. At the moment this only affects the rheology and anything that is touched by the phase functions. A future PR will add density lookup capabilities as well.

At the moment this is a WIP because:

• It breaks some visco_plastic_phases tests because some new code is not avoided in the original non-lookup setup.
• Some assertions are needed to check that users have provided enough information (sufficient information of phases).
• It implicitly assumes the background field uses a lookup table by iterating over all compositions +1.
• It needs additional tests.

Open questions are:

1. Is the structure is correct and consistent with the guidelines?
2. Are there more concise ways to interface with the lookup table information?
3. What are some good testing options?

Many thanks to @naliboff @jdannberg @danieldouglas92 for previous discussions! Any additional feedback from those in the community would be appreciated!

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[gassmoeller]

I need to continue this tomorrow, but a few comments for now. I think the general idea is correct, but we can optimize things a bit and make it easier to understand.

[gassmoeller]

Some more comments, in particular with requests for more documentation. I think I begin to understand what you are doing here, but it will require a few rounds of improvements to get it into a state that is understandable for a non-expert.

[naliboff]

@ryanstoner1 - a few additional comments on top the existing ones, but so far I think the structure you implemented makes sense. At this stage, more documentation in the highlighted places would be helpful to understand what is happening in some places. Thanks for this contribution, it is part of a big step forward in the class of simulations we can do with the visco_plastic model!