Bug fix empty extruded set with variable layers

This would fail if the provided layers array has zero length (e.g.
for boundary facets on an interior parallel domain) as numpy does not
allow you to call min() on such an array.

pyop2/types/set.py

@@ -305,7 +305,7 @@ def __init__(self, parent, layers):
         try:
             layers = utils.verify_reshape(layers, dtypes.IntType, (parent.total_size, 2))
             self.constant_layers = False
-            if layers.min() < 0:
+            if layers.min(initial=0) < 0:
                 raise ex.SizeTypeError("Bottom of layers must be >= 0")
             if any(layers[:, 1] - layers[:, 0] < 1):
                 raise ex.SizeTypeError("Number of layers must be >= 0")