NexusReader classes updated.

Now with working block BEGIN and END/ENDBLOCK searching. As well as
various extraction methods to cope with different brackets (e.g.
comments). New class NexusReaderBlock added to allow different blocks to
be read (currently empty and in need to coding). Also changed the
NexusReaderToken class worked by inputting a QString of all the NEXUS
data rather than a QTextStream object to eliminated excess file reading
(this has decreased the read/parsing time for a 24kb file from 5s to
48ms).