Replace viscoplastic parameter with Enable elasticity

[anne-glerum]

Instead of an additional parameter in the viscoplastic material model, use the parameter Enable elasticity in subsection Formulation to determine whether elasticity should be switched on. Enable elasticity needs to be true in any case to use elasticity in the viscoplastic material model.

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.

[gassmoeller]

Looks good, but this will require changes to parameter files and should therefore also get a changelog entry.

[bangerth]

If the old and new parameter had been in the same section, we could have declared them as aliases: https://dealii.org/developer/doxygen/deal.II/classParameterHandler.html#aecb847561585089c4c7fcf51eddece16 But this doesn't work with parameters in different sections.

[anne-glerum]

If the old and new parameter had been in the same section, we could have declared them as aliases: https://dealii.org/developer/doxygen/deal.II/classParameterHandler.html#aecb847561585089c4c7fcf51eddece16 But this doesn't work with parameters in different sections.

Good to know!