Taal: Rethink file/folder structure

[sloede]

After Taal is feature complete and we have deleted "the old Trixi", but before we merge it to master, we should rethink the file and folder structure to make sure that

• existing stuff is "easy" to find and
• new stuff has a "natural" place where it should be added to.

[ranocha]

I totally agree. Something to think about: Many (some?) editors display the file name without path in a more prominent position than the rest of the path. In that case, it can be much more convenient to include some important information in the file name itself instead of the path, e.g. dg_1d.jl and dg_2d.jl instead of 1d/dg.jl and 2d/dg.jl. In particular, this makes working on 1D and 2D versions in VS Code more convenient for me.