The tag <<CLASS_NOT_PREPARED>> is problematic for the importation in FamixCallStack

[LeoDefossez]

Since this tag is actually a String, and with the current logging it is impossible to differentiate the string reference having the value <<CLASS_NOT_PREPARED>> and a class actually not being prepared.

While being very unlikely to happens, correcting it is important

This could either be corrected during #4 or #5