Merge pull request #641 from OP2/rebase-ext-mesh-fix

Bug fix empty extruded set with variable layers

Author: wence-

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")