Changed BehaviorRoot to a Decorator,

in order to favor composition over inheritance. This makes BehaviorRoot a
pure marker instead of having the mixed responsibility of also acting as a
Composite for other ISpecimenBuilders. When similar changes have also been
applied to other nodes deriving from CompositeSpecimenBuilder, this would
theoretically make it easier to change an entire object graph and, e.g.
replace all Composites with a parallelized version.
Documented new BehaviorRoot members.
Added a Null Guard to BehaviorRoot constructor.
Refactored to generic test class,

in order to hit all marker nodes with the same set of tests, as by
definition, the should all implement the same protocol.
Changed Customization and ResidueCollector nodes

to Decorators, in order to break the inheritance on
CompositeSpecimenBuilder.
Added documentation for new members.
Refactored AutoPropertiesTarget to a Decorator.
Documented new members on AutoPropertiesTarget.