Methods of moments: compatibility fix for MATLAB < R2015a

The mean(…, 'omitnan') syntax does not exist before R2015a. Use nanmean
instead (either provided by the statistics toolbox, or by our fallback under
matlab/missing/stats/).

matlab/method_of_moments/method_of_moments_data_moments.m

@@ -59,7 +59,7 @@ for jm = 1:options_mom_.mom.mom_nbr
         end
     end
     % We replace NaN (due to leads and lags and missing values) with the corresponding mean
-    if isoctave
+    if isoctave || matlab_ver_less_than('8.5')
         dataMoments(jm,1) = nanmean(m_data_tmp);
     else
         dataMoments(jm,1) = mean(m_data_tmp,'omitnan');