new: iterator interface and improved documentation for JKQTPDatastore
reorganization of library (better separation of common code in jkqtpcommon and other code e.g. in jkqtplotter or jkqtmathtext)
Its called by std::isnan(), std::isinf() and std::isfinite() that are
used in the following methods:
JKQTPimagePlot_getImageMin<T>()
JKQTPimagePlot_array2RGBimage<T>()
JKQTPimagePlot_array2image<T>()