1.7.2 Centroid extraction and GUI

• Implemented centroid() for centroid extraction, written by Frans Hermans, as a separate function.
• Wrapped centroid() in qmethod() as a new attribute extraction="centroid" OR "PCA". Test conducted to confirm that the wrapping in qmethod() and the step-by-step manual analysis as shown in the example in ?centroid() are the exact same.
• Added package version to results$brief in both qmethod() and qzscores().
• Added warning notes to uncommon exercises in Q: bootstrapping, FSI indicator, rotations other than varimax.
• Found and reported that the URL for FlashQ doesn't work anymore.
• Enhanced help for qindtest(), explicitly saying it implements the "reordering-reflection" solution from Zabala & Pascual.
• Updated/ improved various help pages as necessary.
• Streamlined data types for 'loadings', 'SD diff' and 'flagged' (#277)--this added several bugs which were then fixed in earlier commits after v1.7.
• Integrated Graphical User Interface (previously external files) as runInterface(), and added centroid option to it.
• Added function to import data from easy-HTMLQ.