Octave compatibility fix: sum(…, 'omitnan') does not exist under Octave

Replace it by nansum from the statistics package.

matlab/evaluate_steady_state_file.m

@@ -79,7 +79,11 @@ end
 
 if updated_params_flag && ~isreal(params1)
     info(1) = 23;
-    info(2) = sum(imag(params).^2,'omitnan');
+    if ~isoctave
+        info(2) = sum(imag(params).^2,'omitnan');
+    else
+        info(2) = nansum(imag(params).^2);
+    end
     if M_.set_auxiliary_variables
         ys = h_set_auxiliary_variables(ys,exo_ss,params);
     end
@@ -114,4 +118,4 @@ if steady_state_checkflag
         info(1) = 22;
         return
     end
-end
+end