Specification class

Provides a base abstract class for search specifications that can be combined using logical operators.

The Specification type exposes the following members:

Method	Description
`is_satisfied_by(self, candidate)`	Verifies whether a `MetadataProperty` satisfies the specification.
`both(self, other)`	Combines two search specifications using the logical AND operator.
`either(self, other)`	Combines two search specifications using the logical OR operator.
`is_not(self)`	Negates the specification.