Add new exceptions

src/main/java/ch/hepia/numeric/Matrix.java

@@ -111,9 +111,15 @@ public class Matrix {
 
     }
 
+    public class notASquareException extends Exception{
+        public notASquareException(){
+            super("The matrix is not a square ! ");
+        }
+    }
+
     public double det() {
         if (!isSquare()) {
-            //throw new Exception("");
+            //throw new notASquareException();
         }
 
         if( nbCols() == 1) {