This model validates
Buildings.Templates.Plants.Controls.StagingRotation.StageChangeCommand
in a configuration with one small unit and two large equally sized
units (component avaStaOneTwo). In response to a
varying flow rate, the variation of the required capacity
chaSta.capReq.y triggers stage change events. The
block
Buildings.Templates.Plants.Controls.Utilities.StageIndex is
used to illustrate how these events translate into a varying plant
stage index idxSta.y.