octave: recognize Python __float__ numeric conversion operator

Examples/octave/operator/example.i

@@ -26,4 +26,9 @@
    ComplexVal __paren__(int j) {
      return ComplexVal($self->re()*j,$self->im()*j);
    }
+
+   double __float__() {
+     return (double) $self->re();
+   }
+
 };

Examples/octave/operator/runme.m

@@ -24,3 +24,7 @@ printf("a(3)= %s\n",disp(a(3)));
 
 # friend operator
 printf("2*a = %s\n",disp(2*a));
+
+# conversions
+printf("single(a) = %g\n", single(a));
+printf("double(a) = %g\n", double(a));