Add childrenType to SPP

src/spp.ts

@@ -21,6 +21,7 @@ export class SPP extends Nub {
         super(prms, dbg);
         this._calcType = CalculatorType.SPP;
         this._defaultCalculatedParam = prms.Y;
+        this._childrenType = "YAXN";
         this.resetDefaultCalculatedParam();
         this.properties.setPropValue("sppOperation", SPPOperation.SUM);
     }