Rename phase transition index

[lhy11009]

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.

Renamed the variable phase_index to phase_transition_index across
affected functions and structures. The new variable name, phase_transition_index,
provides a more precise and descriptive representation of its role,
emphasizing its purpose in indexing phase transitions rather than phases.

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For all pull requests:

• I have followed the instructions for indenting my code.

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• I have tested my new feature locally to ensure it is correct.
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[lhy11009]

Some fact check before progress

• In the tompgraphy_based_plate_motions (both .h and .cc files), the "phase_index" means phase, so no change is applied.
• The diffusion_viscosity (and other similar functions of rheology) in grain_size (both .h and .cc files) means phase
• In the include/aspect/material_model/utilities.h and .cc files

1. All the in.phase_index are changed to in.phase_transition_index
2. All the phase_inputs.phase_index are changed to phase_inputs.phase_transition_index
3. In the member functions of PhaseFunction, all the phase_index are changed to phase_transition_index.
4. In phase_average_value, it means phase.

• In the visco_plastic.cc file

1. All the in.phase_index are changed to in.phase_transition_index
2. All the phase_inputs.phase_index are changed to phase_inputs.phase_transition_index

[gassmoeller]

Very nice, I have a suggestion for shortening the changelog entry, and please squash the two commits into one (or give the second one a better commit message like 'Add changelog').

Afterwards this is good to go.

[gassmoeller]

/rebuild

[lhy11009]

@gassmoeller Thanks for your review, I have committed the change and merged the commits.