pygplib 2.0.1

• Improved documentation.
• Implemented the code for the edge-clique-cover computation.
• Implemented direct-encoding for domain of discourse.
• Removed tools/ and pygplib/util.py to remove dependencies to third-party tools.
• Renamed Fo class to Fog class.
• Updated GrSt class so that
  • object is initialized with a vertex-list and a edge-list, and the ECC computation is performed during the initialization.
  • the methods for the interpretation of relation symbols = and edg as well as compute_domain_constraint() are provided by GrSt, making Fog class and op module independent of domain encoding.
• Changed the format of first-order formulas (negation, existential and universal quantifiers) to make it compatible with TPTP format.
• Renamed decode_assign() of Cnf to decode_assignment() and changed an output assignment so that auxiliary variables are ignored.
• Renamed get_interpretation_of_assign() of GrSt to decode_assignment() and changed an output assignment so that first-order variables' indices are associated with constant symbol indices.
• Removed field st in formula class and changed to give relational structure as argument of each method that requires it.
• Changed API of compute_domain_constraint() so that the input is an index of a free variable and the output is a Prop formula object.