plotlyoffline: fix when filename is returned as a cellstr

In some cases, the filename is obtained as a cellstr. This edit converts to a valid char filename to be used for local storage.

Author: farhi2

plotly/plotly_offline_aux/plotlyoffline.m

@@ -71,6 +71,7 @@
     % template entire script
     offline_script = [dep_script env_script plotly_script]; 
     filename = plotlyfig.PlotOptions.FileName; 
+    if iscellstr(filename), filename = sprintf('%s ', filename{:}); end
 
     % remove the whitespace from the filename
     clean_filename = filename(filename~=' ');