Return user-set maximum topography when the initial topography model Function is asked for the max topography

[anne-glerum]

The maximum topography is read in from the prm for the function initial topography model, but it is not used. This PR returns the user-set maximum topography when the topography model is asked for it. It is not used to restrict topography as given by the function itself, and the user should take care that the value is reasonable.

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For new features/models or changes of existing features:

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

The four failing tests are all tests that include initial topography through the function plugin.

[gassmoeller]

Yes, this was clearly an oversight. Can you update the test output? Afterwards this is good to go.

[anne-glerum]

@gassmoeller I've updated the test results. The failing tests on dealii-master are still all gmg tests with the same error.

[gassmoeller]

Thank you, good to go.