Advection test case

[einola]

We need to decide on an advection test case. Possible choices are:

• A drifting "blob" as implemented in the SI3 test suite
• Zalesak's (1979) "rotating PacMan"
• Richter et al.'s (2023) version of Zalesak's test

I've ordered them by the difficulty of implementation. As always, we should probably aim for the most complete, i.e. difficult case.

[timspainNERSC]

Part of this will be implementing boundary conditions (open, closed, periodic) for the dynamics, as well as the means for the user to set the boundary type.