remove checks for edge cases in boundary repulsion

[drbergman]

• bottom line: at each mechanics time step, it will suffice to push the cell in a cardinal direction to increase the shortest distance to the boundary
• this is precisely what the current code currently does the vast majority of the time.
• now, it will do so for the previously defined edge cases
• in 2D sims, they looked for cells on line y=x
  • then used signs of x and y positions to determine which direction to push
  • no check for cells in upper left or lower right corners
  • the sign of the positions does not accurately convey which way to push (if all corners are negative, they would all push the same way)
• in 3D sims, they looked for cells on line x=y=z
  • similar issues but missing more cases (x=\pm y = \pm z)