Proposal: change the .center() function such that it always respect the axis parameter

Following discussion in the ASE chat: cell axes that are not mentioned in the axis parameter should not be affected by the center function, right now the "about" parameter is breaking this rule.

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.