Fix pierce calculation.

I'm not sure why I'm getting a NumPy array here, but probably once the
lists of arrays goes away, it should fix itself.

obspy/taup/seismic_phase.py

@@ -8,6 +8,8 @@
 from itertools import count
 import math
 
+import numpy as np
+
 from .arrival import Arrival
 from .helper_classes import TauModelError, TimeDist
 
@@ -884,7 +886,7 @@ def calc_pierce_from_arrival(self, currArrival):
                 distB = tauBranch.dist[rayNum + 1]
                 timeB = tauBranch.time[rayNum + 1]
             branchDist += distRatio * (distB - distA) + distA
-            prevBranchTime = branchTime
+            prevBranchTime = np.array(branchTime, copy=True)
             branchTime += distRatio * (timeB - timeA) + timeA
             if isDownGoing:
                 branchDepth = min(tauBranch.botDepth, turnDepth)