fix(PlotMapView): default to all layers in plot_pathline()

Defaulting to layer 0 for arrays where each cell is assigned a color is sensible but imo it is more natural to plot complete pathlines by default

flopy/plot/map.py

@@ -797,7 +797,7 @@ def plot_pathline(self, pl, travel_time=None, **kwargs):
                 else:
                     kon = self.layer
         else:
-            kon = self.layer
+            kon = -1
 
         # configure plot settings
         marker = kwargs.pop("marker", None)