v1.21.0

PowerSystems v1.21.0

Diff since v1.20.2

Merged pull requests:

• Add new AVRs ST1 and EX4VSA (#874) (@rodrigomha)
• Add dera (#875) (@marenat)
• include struct for STAB1 (#877) (@tavovalmo)
• Add genqec rr struct (#878) (@tavovalmo)
• Add Exponential Load model (#880) (@rodrigomha)
• Fix Source R and X source base values (#881) (@rodrigomha)