Fix Gap Used in Common-Plane Timestep Vote

[srwopschall]

• Refactors the gap used in the common plane timestep calculation to reset the existing gap to g=0
• This fix avoids timestep crashes when the existing gap, or the velocity projected gap is slightly beyond the timestep interpenetration amount (as a factor of the element thickness).
• This fix also allows for a soft contact response without a timestep crash in the presence of larger amounts of interpenetration.
• Tests and comments have been updated