Filename conventions for pipeline:

Want to achieve something like: expId.state.classId.ext

where expId is the unique exposure identifier;
      state is the state of processing for that file;
      classId is the class identifier, specifying a particular component of the exposure; and
      ext is an extension indicating the type of file (e.g., fits).

Constraint: classId cannot split components known only by the caller
(pmFPAfile groups all of these into the {OUTPUT} variable).

This constraint does not hinder us, since having the classId as the
last element before the extension makes it simple to get all
components of an exposure at a particular processing state.

Some realisations:

Input is typically:	expId.classId.ext
Detrend processed:	expId.detproc.classId.ext
Detrend residual:	expId.detresid.detId.iter.classId.ext
Stacked detrend:	camera.detType.detId.iter.classId.ext
Phase 1 processed:	expId.p1.classId.ext
Phase 2 processed:	expId.p2.classId.ext
Phase 3 processed:	expId.p3.classId.ext
