Group control can be instituted after the simulation has started, at a well change time or upon
restart. The simplest example consists in introducing a field target after the simulation has run
for some time. Assume that a run has been started with no group structure referred to, i.e. with
wells defined using *WELL lines with no *ATTACHTO subkeywords defining parent groups.
To establish a group structure, it suffices (at a well change time) to introduce the single line