Fix initialization of wdust for dust settling

[jmaerz]

The settling velocity of 'free' dust wdust seem to have not been properly initialized when running the model without the -D AGG flag. Further, some surface input flux could happen, since wdust, correctly initialized, would always be positive. Hence, I introduced wdustd (similar to wpocd, etc.). This bugfix can affect the sediment budget, thus the burrying and sediment remineralization rates.

[TomasTorsvik]

We might want to place this bug fix in a new development branch instead of the master branch.

[jmaerz]

Yes, we can do so. I just wanted to prepare the fix and make you aware of the issue.

[jmaerz]

Argh, I just realized, I was wrong wrt initialization. The issue with the surface flux might be an issue though, which I need to check. Sorry for the confusion.

[jmaerz]

I reverted changes related to the initialization. However, the fix for the surface flux of free dust remains, and with that the potential change of sediment-related processes.

[JorgSchwinger]

Thanks for spotting this, looks good. I agree with Tomas that we shouldn't merge this into master right now. We should discuss this very soon, and make a plan for when to start merging answer changing changes (as raised by Tomas in #169).