Added documentation

A first draft of the documentation of the DataError class was added.
Added initialization to DataError

New tests (and the implementation) were added to the class to
initialize a DataError of a chain of errors.
Changed the name of the library in some comments

Well, copy-and-pasting produces this nasty effects. A hundred of times
must be repeated the statement: "I won't copy-and-paste any more". That
is the penalty...