0.3.0
- add FactorMatch NamedTuples to be listed in Explanation
- remove FactorGroup.comparison method. Instead use implies, contradicts, etc.
- Factor "explanations" methods yield Explanation, not ContextRegister
- add FactorGroup.explanations_implied_by method
- fix bug: shares_all_factors passed without checking all factors
- fix bug: two empty FactorGroups should have same meaning