@dseries/{first,last}observedperiod.m: MATLAB R2009b compatibility fix

prod() does not accept logical arrays, need to explicitly convert to doubles.

src/@dseries/firstobservedperiod.m

@@ -26,7 +26,7 @@ function d = firstobservedperiod(o) % --*-- Unitary tests --*--
 % along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
 
 b = ~isnan(o);
-c = find(prod(b, 2));
+c = find(prod(double(b), 2));
 
 if isempty(c)
     error('No overlapping non-NaN data points found in dseries.');

src/@dseries/lastobservedperiod.m

@@ -26,7 +26,7 @@ function d = lastobservedperiod(o) % --*-- Unitary tests --*--
 % along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
 
 b = ~isnan(o);
-c = find(prod(b, 2));
+c = find(prod(double(b), 2));
 if isempty(c)
     error('No overlapping non-NaN data points found in dseries.');
 end