move viscosity averaging function

[bobmyhill]

This PR moves the get_averaging_operation_for_viscosity() function from material_model/rheology/elasticity.cc into material_model/interface.cc.

This is a more logical place for the function, and makes it more easily accessible by rheology and material_model modules.

Addresses a single comment here: #5984 (comment)

[bobmyhill]

/rebuild

[bobmyhill]

/rebuild

[gassmoeller]

would aspect/material_model/utilities.h be a better place? you only want to make this accessible to other material models than just elasticity, correct?

[bobmyhill]

@gassmoeller that is probably true, but it would require more code refactoring. interface.h contains the definition of the AveragingOperation enum, and also includes <aspect/material_model/utilities.h>. So to avoid circular dependencies I would have to move the enum definition and possibly some of the other functions over to utilities.

I'm happy to either stick with what I have here or do the refactoring.

[gassmoeller]

Ah, I see. I am ok with keeping it in the interface for now then. However, please extend the documentation of the function similar to what I suggest.

[gassmoeller]

Please extend the description of the function, I had to read the implementation to understand what is happening here. Something like the following:

       * Parse an AveragingOperation and alias to an AveragingOperation
       * that only averages viscosity. In the case of any averaging operation 
       * that averages all properties the corresponding operation for only
       * viscosity is selected (e.g. 'harmonic_average_only_viscosity' instead
       * of 'harmonic_average'). This is useful in places where averaging is
       * performed manually on only the viscosity property.

[gassmoeller]

👍 good to go if the testers are happy.