Added second output argument to setdiff method.

If o and p are two dates objects, then with

[q, io] = setdiff(o, p)

we obtain in q the elements belonging to o but not to p, and io is such
that q = o(io).

src/@dates/setdiff.m

-function q = setdiff(o,p) % --*-- Unitary tests --*--
+function [q, io] = setdiff(o,p) % --*-- Unitary tests --*--
 
 % Overloads setdiff function for dates objects.
 %
@@ -7,7 +7,8 @@ function q = setdiff(o,p) % --*-- Unitary tests --*--
 % - p  [dates]
 %
 % OUTPUTS 
-% - q [dates]
+% - q  [dates]   with n elements
+% - io [integer] n*1 vector of integers such that q = o(io)
 %
 % See also pop, remove.
 
@@ -36,26 +37,32 @@ end
 
 if isempty(p)
     q = copy(o);
+    if nargout>1, io = 1:length(q); end
     return
 end
 
 if o==p
     % Return an empty dates object.
     q = dates(o.freq);
+    if nargout>1, io = []; end
     return
 end
 
 if isoctave || matlab_ver_less_than('8.1.0')
+    if nargout<2
         time = setdiff(o.time,p.time,'rows');
     else
+        [time, io] = setdiff(o.time,p.time,'rows');
+    end
+else
+    if nargout<2
         time = setdiff(o.time,p.time,'rows','legacy');
+    else
+        [time, io] = setdiff(o.time,p.time,'rows','legacy');
     end
-
-q = dates(o.freq);
-if isempty(time)
-    return
 end
 
+q = dates(o.freq);
 q.time = time;
 
 %@test:1