Improve comment.
Simplify __eq__; drop __ne__

Implementing both __eq__ and __ne__ is error prone.
As it should always be the case that
    (a != b) == not (a == b)
only one magic function is needed; the other will be provided
automatically.
A simple hash.
Implement __repr__; derive __str__ automatically.

Implementing __str__ but not __repr__ is considered bad practice,
and makes debugging a bit awkward, because Python drops in a default
replacement for the missing __repr__. However, if only __repr__ is
implemented, then it will be used for both repr and str calls.
Unit tests for node/contact.py.
Remove execute flag.
Fix docstring.