Implement add() to add cnfvars to cnfvar lists directly from tuples

Implement add() to add cnfvars to cnfvar lists directly from tuples

The CnfList class already has some functionality for initialization
directly from tuples and the Cnf class already has some functionality
like add_child() for adding a cnfvar from a tuple to Cnf instance's
CnfList instance children. To complete the syntactic sugar circle,
also provide the further possibility to add a CnfList item straight
from a tuple using add() instead of the more raw and manual append()
which can also handle both Cnf instances and tuples similarly to the
previous two options above.