Blend-in aero-acoustic source term

[jh66637]

As indicated in #635 a few small follow-up PRs are necessary to use the aero-acoustic solver in realistic settings.

This PR introduces a way to blend in source terms. Mostly this blend-in is performed in time: I.e. the CFD solver is running and at some point, the acoustic solver starts to run as well. To avoid artificial waves due to the sudden application of the source term these source terms are typically slowly increased over time in the aero-acoustic community.

While this is mostly enough and I didn't have problems in my test cases, in literature it is often reported that source terms are spatially faded out by a blending as well. I.e., to avoid large gradients at CFD mesh boundaries. The CFD mesh is typically smaller than the acoustic mesh and both meshes are coupled arbitrarily, so one can imagine this becoming a potential error source.

[nfehn]

I had a detailed look at the code. I understood the basic idea. However, I think we need to improve readability of the implementation, i.e. how the source term gets multiplied by a blending function, and the documentation.

[jh66637]

You raised some good questions. Let's decide how we proceed with #644 (comment) and #644 (comment) before I resolve all the issues :)

[jh66637]

I think I addressed all the comments.

[jh66637]

@nfehn I rebased to the latest version of master. The failing pipeline is not related to this PR.

[nfehn]

I will look over the changes again.

The failing pipeline comes from deal.II, but this should now already be fixed in ExaDG.

[jh66637]

@nfehn I think I addressed all your concerns. Let me know if this needs more work to get it merged :)

[nfehn]

Great! I see nothing that would hinder us from merging :)