Model issues detected by unit checking ThreePhaseTwoLevel_PWM

[HansOlsson]

Might add more later, but a first one is that Modelica.Electrical.PowerConverters.DCAC.Control.SVPWM used in Modelica.Electrical.PowerConverters.Examples.DCAC.PolyphaseTwoLevel.ThreePhaseTwoLevel_PWM has incorrect start-value for t0 (not only wrong unit - but wrong value).

The relevant parts are:

 discrete Real t0(start=samplePeriod, fixed=true) "Relative time spans of vectors a, b, and 0";
algorithm
  when sampleTrigger then
     t0=max(1-ta-tb,0);
   end when;
...
   elseif (time - T0)/samplePeriod < (t0/4) then
...

It is clear from both of the latter two equations as well as the description that t0 should have unit="1", as it is a relative time-span.
That is not consistent with the start-value samplePeriod, and I would guess that a better start-value is 1.

(I would also prefer rewriting the elseif-conditions as elseif time < T0+samplePeriod*(t0/4) then since that is one of the forms that is discussed for time-events.)
Note that this will only matter in weird cases, as it is only used after startTime and before the first triggering of sampleTrigger.

(This is in addition to all the other related issues with PR.)