Refactor/advection

[timfelle]

• Simplified the factory.
• Reduced usage to only whats used.
• Split advection into multiple files.

[timofeymukha]

Very nice. What I am wondering, now that we are restructuring a bit, is whether we should have this in fluid and the different advection types in bcknd, moving forward. It is used in scalar as well, I suppose. I think we could promote it to its own directory in src. Something for you guys to consider.

[njansson]

I guess a merge with develop will resolve the failed builds