Assign particle to a group
If the style region is used, particle within this region at the beginning will be assigned to this group.
If the style type is used, particle of this type will be assigned to this group.
Before every run command, particle list in this group will be updated.
Whether group all is specified in the input script or not, the first group is always all which means all particles belong to the first group.
The maximum number of group can be created is 31.