DPStokes (periodic, periodic, open) needs net zero force on the domain

[rykerfish]

This article explains how the diffusion coefficient diverges in doubly periodic systems with the third dimension being open. As a result, the net force on the domain needs to be zero when DPStokes is open in z.

TODOs:

• add a check in DPStokes when it is open in $z$ to ensure the net force on the domain is zero.
• add a flag to DPStokes to avoid this check and document that it is unsafe but can be done.