Support Partial Refact with Reduced-Order VBR Models as Current Source

[dinkelbachjan]

The varying entries need to be set properly for this, see for example here:

dpsim/dpsim-models/src/DP/DP_Ph1_ReducedOrderSynchronGeneratorVBR.cpp

Lines 89 to 91 in 758d47c

	if (mModelAsCurrentSource) {
	// FIXME set variable matrix entries accordingly as shown below
	mVariableSystemMatrixEntries.push_back(std::make_pair<UInt,UInt>(matrixNodeIndex(0, 0), matrixNodeIndex(0, 0)));