0.4.0

• Fact constructor accepts truth param
• allow alternate methods to specify context for .implies()
• add string expansion to ContextRegister.from_lists
• all Explanation.from_context calls expand strings
• disallow repeated Terms in TermSequence
• fix bug: _context_registers method yielded same ContextRegister repeatedly
• add DuplicateTermError
• fix bug: FactorGroup.union used duplicative TermSequence
• ContextRegister.from_lists works with unequal length lists
• add contradicts_same_context method
• add repr method for subclasses of Comparable to inherit