Indexers are now cloned if needed to modify them after they've been

closed. Obligatory indexers' names now follow the same pattern as other
indexers (columnname_version). Closes #46 and #49.

src/obierrno.h

@@ -114,7 +114,9 @@ extern int obi_errno;
 	 	 	 	 	 	 	 	 	 	 	 */
 #define OBI_INDEXER_ERROR			(27)    /** Error handling a blob indexer
 											 */
-#define OBI_ALIGN_ERROR				(28)    /** Error while aligning sequences
+#define OBI_READ_ONLY_INDEXER_ERROR (28)    /** Error trying to modify a read-only a blob indexer
+											 */
+#define OBI_ALIGN_ERROR				(29)    /** Error while aligning sequences
  	 	 	 	 	 	 	 	 	 	 	 */
 /**@}*/