Modifying 3-level modulation

Previous PLECS simulations to mimic the shifted carrier strategy
presented in  Josep Pou's 2012 Transactions on Power Electronics
paper were wrong. This revision doesn't scale the modulation
indices by 0.5 followed by +0.5 shift. Instead modulation in its
original [-1 1] form is given to SetPWM_Na1((dra)*PWM_PD); and
a +1.0 shift dra signal is given to SetPWM_Na2((dra+1.0)*PWM_PD);

PLECS simulations trying to match the PLECS "Symmetrical PWM (3-leve)"
block for the "Negative carrier: Shifted" proved to match the paper
rather than the "Negative carrier: flipped".

Hopefully this commit will lead to a working NPC 3-level rectifier
with zero-sequence injection described by Josep Pou!