Wrote Group.insert() test.
Wrote Group.__setitem__() test.
Improved Group.__setitem__() and tested Node.precede().
Wrote Node.succede() test.
Wrote Node.replace() test.
Wrote Node.parentage test.
Renamed Node.replace() to Node.replace_with().
Renamed Node.precede() to Node.precede_by().
Renamed Node.succede() to Node.succede_by().