allow a different list of assemblers for each advection field

[jdannberg]

It is useful to allow different advection fields to have different assemblers (i.e., we have different advection methods like advection field, diffusion field etc.) and not all of them require all assemblers. In cases where a signal is used to replace assemblers, it may even be necessary to only do that for specific fields and not for all of them.

This PR makes the advection system assembers a vector of vectors (with each compositional field having their individual vector of advection assemblers). At the moment, all fields still get the same assemblers, to test that this change does not break any functionality. The next step would be to only include the assemblers that are needed in each list (in a separate PR).

The change in the entropy benchmark shows how this could look like.

I also want to see which tests this breaks to see where I still have to fix the assemblers.

For all pull requests:

• I have followed the instructions for indenting my code.
• I have added a changelog entry in the doc/modules/changes directory that will inform other users of my change.

[gassmoeller]

The code looks correct, but lets see what the testers say.

[jdannberg]

The tester thinks it's correct 🙂