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

WHY IS THIS CHANGE NEEDED?:
- detrend process overzealously removes columns if input dataframes
  have NaNs near the beginning or end.
- this is because by default, df.dropna() uses how="any", i.e. it drops
  columns with at least one NaN

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- drops columns with at least one NaN is not the intention -- the
  intention is to remove column filled with NaNs created by rolling
  window operations.
- so, specified the how argument

WHAT SIDE EFFECTS DOES THIS CHANGE HAVE?:
- i honestly don't know why i didn't catch it in the 1.5 years this
  process existed -- this seems so obvious.
- this is a deprecated signal processing post-process (i use the
  butterworth filter instead), so i don't expect it to break things much

EVIDENCE THAT COMMIT WORKS: