New statements in AnnotationFile to ensure single character column annotation can be exported/re-imported

From JAL-2313
Exporting and reimporting the results of RNAalifold via an alignment annotation file yields an alignment view where the reference position annotation row (below) is interpreted as RNA secondary structure rather than single character column labels.

NO_GRAPH Reference Positions Reference Positions g|.|u|c|u|c|a|u|u|.|.|u|A|U|G|A|u|G|g|c|c|u|c|u|c|c|c|.|u|A|A|A|.|u|c|c|c|u|u|u|.|.|.|g|g|g|.|.|c|.|.|.|.|.|.|.|.|g|g|g|a|g|a|.|g|.|g|.|c|C|u|G|A|U|.|.|g|c|u|u|g|.|.|.|.|.|.|.|g|a|g|a|c|

Simplest solution is to introduce a new annotation row type string that instructs the parser to interpret the annotation row definition as labels only - such as 'LABELS'.