Refactor predicate

Reviewers: @cdescours

Added accessing value function

• DatabaseModel description
• currentValue to current for DbModel parent interface protocol

Refactored Predicates

• Moved the logic of ComparisonPredicate and OperatorPredicate into a single Predicate struct.
• This allows to specify a predicate with a single function rather than several overloadings.
• The where parameter has been moved to a .where(:) extension on Predicate