fixed some findbugs issues

src/main/java/de/neemann/digital/core/Model.java

@@ -430,6 +430,22 @@ public class Model {
             return name.compareTo(o.name);
         }
 
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) return true;
+            if (o == null || getClass() != o.getClass()) return false;
+
+            Signal signal = (Signal) o;
+
+            return name.equals(signal.name);
+
+        }
+
+        @Override
+        public int hashCode() {
+            return name.hashCode();
+        }
+
         @Override
         public String toString() {
             return name;

src/main/java/de/neemann/digital/draw/graphics/GraphicSVG.java

@@ -93,7 +93,7 @@ public class GraphicSVG implements Graphic, Closeable {
     public void drawCircle(Vector p1, Vector p2, Style style) {
         try {
             Vector c = p1.add(p2).div(2);
-            double r = Math.abs(p2.sub(p1).x) / 2;
+            double r = Math.abs(p2.sub(p1).x) / 2.0;
             if (style.isFilled())
                 w.write("<circle cx=\"" + c.x + "\" cy=\"" + c.y + "\" r=\"" + r + "\" stroke=\"" + getColor(style) + "\" stroke-width=\"" + getStrokeWidth(style) + "\" fill=\"" + getColor(style) + "\" />\n");
             else {