added mammal class

ZooProgram/src/Mammal.java

+public class Mammal extends Animal{
+
+    public Mammal() {
+        super();
+    }
+
+    public Mammal(String name, String description, String environment, String foodType, Double weight, Double height){
+        super(name,description,environment,foodType);
+        addDetail("Weight: "+weight.toString());
+        addDetail("Height: "+height.toString());
+    }
+}