Definition of step in PCS description for reference tracking tests

[louisg-rte]

In the PCS description the reference tracking tests are defined as follow:

The parameter setpoint_step_value (line 66) is the step size (final value - initial value) of the setpoint but its initial value is not configurable and is defined (I guess) as the initial voltage at PDR which is here pdr_U (line 49).
We should add a parameter that corresponds to the initial value of the setpoint. The reason is that, depending on the activated control mode, we don't necessary have U = Uref (even in the steady state before the event). Indeed a typical control mode for WT is Uref = U + lambda Q (lambda being a fixed parameter). (<=>Mwpqmode = 3 in the IEC WPP model). If we decide to define Q ≠ 0 at the initialization, we'll have U≠Uref, so the intial value of Uref should be configurable.