Relax MPCD restrictions on boxes

[mphoward]

Description

MPCD currently requires that the box size be (1) orthorhombic, (2) constant, and (3) a multiple of the MPCD cell size. Only requirement (3) is strictly necessary for the algorithm to work right, so I will check over the code to see if it makes sense to support other (e.g., deformed) boxes. This is change is not necessarily breaking but may not be the behavior expected by the user, so I will look into it for v3.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[github-actions]

This issue has been automatically closed because it has not had recent activity.

[mphoward]

We will need to address this as part of our new work.