#187 NexML character matrix contains {}

[Thasso Griebel]

The matrix rows in nexml files contain nexus style ambiguities. For example {AT} instead of W. I am not sure if this is allowed, but the validator reports the files as invalid. Also note that there is a bug in the nexml library ( https://sourceforge.net/tracker/?func=detail&aid=3037220&group_id=209571&atid=1010249 ), so the ambiguity states for Y and H are wron in the resulting nexml file.