MAX_CRIT group criterion needs re-implementation

[lplana]

The MAX_CRIT group criterion is described in the Lens Manual.

This group criterion, implemented in function max_stop_crit() [file process_t.c] needs re-writing as the current implementation only works correctly if only one unit has the largest target, which is not usually the case.

Additionally, as this criterion is based on group-wide values rather than on individual unit ones, the function needs to make a correct distributed decision when the output group is split across multiple subgroups.