fix(postproc): crosscorr catches case where 2nd trace is defined

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

src/postprocessor/core/multisignal/crosscorr.py

@@ -115,6 +115,8 @@ class crosscorr(PostProcessABC):
         if trace_dfB is None:
             trace_dfB = trace_dfA
             trace_B = trace_A
+        else:
+            trace_B = trace_dfB.to_numpy()
         # find deviation from the mean
         dmean_A, stdA = _dev(trace_A, n_replicates, n_tps, self.stationary)
         dmean_B, stdB = _dev(trace_B, n_replicates, n_tps, self.stationary)