Merge strategy DFP

This merge strategy implements the algorithm originally described in the paper "Directed model checking with distance-preserving abstractions" by Draeger, Finkbeiner and Podelski (SPIN 2006), adapted to planning in the following paper:

• Silvan Sievers, Martin Wehrle, and Malte Helmert.
  Generalized Label Reduction for Merge-and-Shrink Heuristics.
  In Proceedings of the 28th AAAI Conference on Artificial Intelligence (AAAI 2014), pp. 2358-2366. AAAI Press 2014.

merge_dfp()

Linear merge strategies

This merge strategy implements several linear merge orders, which are described in the paper:

• Malte Helmert, Patrik Haslum and Joerg Hoffmann.
  Flexible Abstraction Heuristics for Optimal Sequential Planning
  In Proceedings of the Seventeenth International Conference on Automated Planning and Scheduling (ICAPS 2007), pp. 176-183. 2007.

merge_linear(variable_order=CG_GOAL_LEVEL)

• variable_order ({CG_GOAL_LEVEL, CG_GOAL_RANDOM, GOAL_CG_LEVEL, RANDOM, LEVEL, REVERSE_LEVEL}): the order in which atomic transition systems are merged

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