Adding a new constraint regarding the ROCOF and frequency, q_RateOfChangeOfFrequency
I think this is a very useful equation. I was just wondering that maybe this could be changed so that it would apply to a group of nodes similar to q_inertiaMin, for example, to make it possible to model ROI+NI using two nodes but applying only one ROCOF constraint for the whole island. Then the parameters would be p_groupPolicy(group, 'ROCOF') and p_groupPolicy(group, 'defaultFrequency'). Additional conditions would be needed when summing kinetic/rotational energy over gnu_output: gnGroup(grid, node, group) and gnuft(grid, node, unit, f, t). Variable v_gen on the right hand side would then also need a gnGroup(grid, node, group) condition and maybe also gnuft(grid, node, unit, f, t).