Make the protected object public to use its functions elsewhere.

[alarshi]

We use the object slab_boundary in our material model to access slab_depths and slab_thickness. In order to do this, this PR allows public access for the object.

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[bangerth]

If you make it public, you also allow other places in the code to change the variable.

My preference would be to keep it protected and instead add an accessor function

  const Utilities::AsciiDataBoundary<dim> &get_slab_boundary() const;

This way, other places can access the data, but because they only get a const reference, they can't change it.

Separate question: Why was it protected to begin with, rather than private?

[alarshi]

Thank you for the review! :) I addressed your comments in the latest commit.
I think Protected access instead of Private was a carryover mistake. I have corrected it now.

[bangerth]

Yes, looks good. Thank you!