Fix #123

 * CT_ChartSpace.sers now just read oxml
 * add method _sort_and_correct_order_sers
Fix #123 : Mode property CT_ChartSpace.sers oxml readonly and compatible

 * sers sorted by `idx`
 * add method reindex_sers (support reindex as previous behavior)
 * add method reset_order_sers (avoid duplicate `order`
 * compatibility for _SeriesRewriter
Merge branch 'proposed_issue123' into chart_cat_multiLvl
chart:CT_NumCache.tuple_pts return (idx, float) istead of (idx, text)