Delegating action invocation to an interface.

This is the first step in a refactoring effort to get rid of the Action<T,
ISpecimenContext>-based API.
Removed some curlies.
Added an ISpecimenCommand property and ctor

to Postprocessor, as a step towards getting rid of the Action-based API.
Saved builder in class field from new ctor.
Saved action field from new ctor.
Saved to specification field from new ctor.
Added Null Guard for builder in new ctor.
Added Null Guard for command in new ctor.
Saved to builder field from full ctor.
Saved to command field from full ctor.
Saved to specification field from full ctor.
Saved to action field from full ctor.
Added Null Guard for builder in full ctor.
Added Null Guard for command for full ctor.
Added Null Guard for specification to full ctor.
Chained ctors.
Added minimal ctor to non-generic Postprocessor,

and assigned builder field.
Passed command to base from non-generic min ctor
Added new full ctor to non-generic Postprocessor

and passed the builder argument to base.
Passed command to base.