Fixed incorrect GCR calculation.

src/ac/ed/lurg/solar/SolarPotentialItem.java

@@ -38,7 +38,7 @@ public class SolarPotentialItem implements RasterItem {
     public void calcOptimalGCR(double latitude) {
         // https://doi.org/10.1016/j.solener.2023.04.038
         // Optimal panel spacing and therefore Ground Coverage ratio depends on latitude
-        this.optimalGCR = -0.55 / (1 + Math.exp(-0.138 * (latitude - 43.4))) + 0.71;
+        this.optimalGCR = -0.55 / (1 + Math.exp(-0.138 * (Math.abs(latitude) - 43.4))) + 0.71;
         this.agrivoltaicsGCR = ModelConfig.AV_GCR;
     }