REP: add main structure of parte2/relazione.tex.
REP: add detailed description of classes.
REP: add description of used threads.
REP: add description of join() construct.
DOCS: add description of commit messages.
REP: add stub of algorithm.
REP: add description of changes.

Add description of how code has changed to make it concurrent, compared
to the sequential implementation.
REP: add TODO in class descriptions.