Changed the behaviour of the sort method...

... and added a new method.

sort_ sorts a dates object in place (without a copy).
sort  first make a copy and then sorts the copy.

The first routine, with in place modification, is more efficient because
we do not have to create a new object.

Example
=======

If o is a dates object with more than one element, then

o.sort()

returns a sorted version of o without modifying o, while

o.sort_()

sorts the elements in o.

src/@dates/sort.m

 function o = sort(o) % --*-- Unitary tests --*--
 
-% Sort method for dates class.
+% Sort method for dates class (with copy).
 %
 % INPUTS 
 % - o [dates]
@@ -23,33 +23,8 @@ function o = sort(o) % --*-- Unitary tests --*--
 % You should have received a copy of the GNU General Public License
 % along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
 
-if isequal(o.ndat(),1)
-    return
-end
-
-o.time = sortrows(o.time,[1,2]);
-
-%@test:1
-%$ % Define some dates
-%$ B1 = '1953Q4';
-%$ B2 = '1950Q2';
-%$ B3 = '1950Q1';
-%$ B4 = '1945Q3';
-%$
-%$ % Define expected results.
-%$ e.time = [1945 3; 1950 1; 1950 2; 1953 4];
-%$ e.freq = 4;
-%$
-%$ % Call the tested routine.
-%$ d = dates(B1,B2,B3,B4);
-%$ d = d.sort;
-%$ 
-%$ % Check the results.
-%$ t(1) = dassert(d.time,e.time);
-%$ t(2) = dassert(d.freq,e.freq);
-%$ t(3) = size(e.time,1) == d.ndat();
-%$ T = all(t);
-%@eof:1
+o = copy(o);
+o.sort_();
 
 %@test:1
 %$ % Define some dates
@@ -61,14 +36,16 @@ o.time = sortrows(o.time,[1,2]);
 %$ % Define expected results.
 %$ e.time = [1945 3; 1950 1; 1950 2; 1953 4];
 %$ e.freq = 4;
+%$ f.time = [1953 4; 1950 2; 1950 1; 1945 3];
 %$
 %$ % Call the tested routine.
 %$ d = dates(B1,B2,B3,B4);
-%$ d = d.sort();
+%$ c = d.sort();
 %$ 
 %$ % Check the results.
-%$ t(1) = dassert(d.time,e.time);
-%$ t(2) = dassert(d.freq,e.freq);
-%$ t(3) = size(e.time,1) == d.ndat();
+%$ t(1) = ~dassert(d.time,e.time);
+%$ t(2) = dassert(c.time,e.time);
+%$ t(3) = dassert(d.freq,e.freq);
+%$ t(4) = dassert(c.freq,e.freq);
 %$ T = all(t);
 %@eof:1
\ No newline at end of file

src/@dates/sort_.m

+function o = sort_(o) % --*-- Unitary tests --*--
+
+% Sort method for dates class (in place modification).
+%
+% INPUTS 
+% - o [dates]
+%
+% OUTPUTS 
+% - o [dates] with dates sorted by increasing order.
+
+% Copyright (C) 2015 Dynare Team
+%
+% This code is free software: you can redistribute it and/or modify
+% it under the terms of the GNU General Public License as published by
+% the Free Software Foundation, either version 3 of the License, or
+% (at your option) any later version.
+%
+% Dynare dates submodule is distributed in the hope that it will be useful,
+% but WITHOUT ANY WARRANTY; without even the implied warranty of
+% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+% GNU General Public License for more details.
+%
+% You should have received a copy of the GNU General Public License
+% along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
+
+if isequal(o.ndat(),1)
+    return
+end
+
+o.time = sortrows(o.time,[1,2]);
+
+%@test:1
+%$ % Define some dates
+%$ B1 = '1953Q4';
+%$ B2 = '1950Q2';
+%$ B3 = '1950Q1';
+%$ B4 = '1945Q3';
+%$
+%$ % Define expected results.
+%$ e.time = [1945 3; 1950 1; 1950 2; 1953 4];
+%$ e.freq = 4;
+%$
+%$ % Call the tested routine.
+%$ d = dates(B1,B2,B3,B4);
+%$ d.sort_();
+%$ 
+%$ % Check the results.
+%$ t(1) = dassert(d.time,e.time);
+%$ t(2) = dassert(d.freq,e.freq);
+%$ T = all(t);
+%@eof:1
+
+%@test:2
+%$ % Define some dates
+%$ B1 = '1953Q4';
+%$ B2 = '1950Q2';
+%$ B3 = '1950Q1';
+%$ B4 = '1945Q3';
+%$
+%$ % Define expected results.
+%$ e.time = [1945 3; 1950 1; 1950 2; 1953 4];
+%$ e.freq = 4;
+%$
+%$ % Call the tested routine.
+%$ d = dates(B1,B2,B3,B4);
+%$ c = sort_(d);
+%$ 
+%$ % Check the results.
+%$ t(1) = dassert(d.time,e.time);
+%$ t(2) = dassert(d.freq,e.freq);
+%$ T = all(t);
+%@eof:2
\ No newline at end of file

src/@dates/subsref.m

@@ -36,7 +36,7 @@ switch S(1).type
             error(['dates::subsref: ' S(1).subs ' is not a method but a member!'])
         end
         B = builtin('subsref', A, S(1));
-      case {'sort','unique','double','isempty','length','char','ndat'}% Public methods (without input arguments)
+      case {'sort','sort_','unique','double','isempty','length','char','ndat'}% Public methods (without input arguments)
         B = feval(S(1).subs,A);
         if length(S)>1 && isequal(S(2).type,'()') && isempty(S(2).subs)
            S = shiftS(S,1);