Add a "mode" that is able to perform load shedding to avoid total blackout

[BDonnot]

Is your feature request related to a problem? Please describe.

This could be another area of measure, especially for "early" stage of training of agent, where you kind of have a "safety net" that is able to "cut" some loads to ensure the grid can continue as always (with of course a rather huge penalty if some loads is cut from the grid)