Instructs the NxsCharactersBlock to convert sequence data character blocks that have "augmented" symbols lists into a mixture datatype.
By default or after SetConvertAugmentedToMixed(false), the NxsCharacterBlock will change the datatype to standard (to indicate that the datatype is no longer simply a sequence type).
Note that GetOriginalDataType() will still store the name of the type that occurred in the file.
This is only applicable if SetAllowAugmentingOfSequenceSymbols(true) has been called. Dealing with Datatype=Mixed