feat(postproc): crosscorr has stationary option

See merge request aliby/aliby!18

WHY IS THIS CHANGE NEEDED?:
- https://gitlab.com/aliby/aliby/-/jobs/4743945774 complains
  "UnboundLocalError: local variable 'trace_B' referenced before
  assignment".
- i had forgotten the case where trace_B is defined

WHY IS THIS CHANGE NEEDED?:
- crosscorr process does not have stationary option and therefore will
  not perform calculations correctly if all signals are in-phase.

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- Additional 'stationary' parameter that tells the process to compute
  the mean over replicates AND time points.

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- crosscorr process not used anywhere else in aliby, so should not
  affect aliby.
- crosscorr process used in multiple places in skeletons (particularly
  arin's projects).  this is an *additional* parameter, and the existing
  behaviour of crosscorr process is same as stationary=False, so i don't
  expect breaking changes.

EVIDENCE THAT COMMIT WORKS:
- new test to be run

REFERENCES:
- #72