(/=) Simplify PhysicalTableInterface

PhysicalTableInterface: succ, min and get are not needed, can be
reduced to iterating with the iterator. Mark some operations as
„optional“ to make it possible that the implementing table is static or
doesn’t provide direct access by key.
(/=) Update SQLOperator: use same semantic as PhysicalTableInterface
for next
(-) OperatorDummy: superseded by StaticPhysicalTable
(/=) TableIterator: make it an interface, remove unnecessary methods
(for now).