Added lineartrend method.

src/@dseries/lineartrend.m

+function trend = lineartrend(o) % --*-- Unitary tests --*--
+
+% Returns a trend centered on zero.
+%
+% INPUTS
+% - o       [dseries]   time series with T observations.
+%
+% OUTPUTS
+% - trend   [double]    T*1 vector, linear trend with mean zero. 
+%
+% REMARKS
+% the generic element of the returned argument is trend(i) = i - (n+1)/2.    
+
+% Copyright (C) 2017 Dynare Team
+%
+% This file is part of Dynare.
+%
+% Dynare 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 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/>.
+
+trend = transpose(1:o.nobs)-.5*(o.nobs+1);
+
+return
+
+%@test:1
+a = dseries(randn(100,3));
+try
+    trend = o.lineartrend;
+    t(1) = 1;
+catch
+    t(1) = 0;
+end
+
+if t(1)
+    t(2) = abs(mean(trend))<1e-12;
+end
+T = all(t);
+%@eof:1
\ No newline at end of file

src/@dseries/subsref.m

@@ -70,7 +70,7 @@ switch S(1).type
             error(['dseries::subsref: ' S(1).subs ' is not a method but a member!'])
         end
         B = builtin('subsref', A, S(1));
-      case {'log','log_','exp','exp_','ygrowth','ygrowth_','qgrowth','qgrowth_','ydiff','ydiff_','qdiff','qdiff_','abs','abs_','isnan','firstdate','firstobservedperiod','lastobservedperiod'}  % Give "dot access" to public methods without args.
+      case {'log','log_','exp','exp_','ygrowth','ygrowth_','qgrowth','qgrowth_','ydiff','ydiff_','qdiff','qdiff_','abs','abs_','isnan','firstdate','firstobservedperiod','lastobservedperiod','lineartrend'}  % Give "dot access" to public methods without args.
         B = feval(S(1).subs,A);
         if length(S)>1 && isequal(S(2).type,'()') && isempty(S(2).subs)
             S = shiftS(S,1);