Landmarks without PDDL domain

[TathagataChakraborti]

Currently, landmark-based selection choices are generated according to the theoretical definition of what is a disjunctive action landmark in classical planning problem. We can relax this assumption to any input set of plans.

• new choice generation algorithm that gives that best set of disjunctive choices for an input set of plans
• relax inputs on frontend so we don't need problem and domain file as necessary (so if problem and domain are there then input plans are optional OR only input plans are provided).