fix empty boundary of SimplexTopology

The constructor of `SimplexTopology` trips over

    assert numpy.greater(self.simplices[:, 1:], self.simplices[:, :-1]).all()

if there are no simplices. This is triggered when taking the boundary of
a fully periodic mesh.

This patch fixes the problem by returning an `EmptyTopology` when the
boundary is empty. In addition this patch adds a slightly improved error
message when trying to create an empty `SimplexTopology.

nutils/topology.py

@@ -2362,6 +2362,7 @@ class SimplexTopology(TransformChainsTopology):
     def __init__(self, space: str, simplices: numpy.ndarray, transforms: transformseq.Transforms, opposites: transformseq.Transforms):
         assert isinstance(space, str), f'space={space!r}'
         assert isinstance(simplices, numpy.ndarray), f'simplices={simplices!r}'
+        assert len(simplices), f'simplices is empty'
         assert simplices.shape == (len(transforms), transforms.fromdims+1)
         self.simplices = numpy.asarray(simplices)
         assert numpy.greater(self.simplices[:, 1:], self.simplices[:, :-1]).all(), 'nodes should be sorted'
@@ -2388,6 +2389,8 @@ def take_unchecked(self, indices):
     def boundary(self):
         space, = self.spaces
         ielem, iedge = (self.connectivity == -1).nonzero()
+        if len(ielem) == 0:
+            return self.empty_like().boundary
         nd = self.ndims
         edges = numpy.arange(nd+1).repeat(nd).reshape(nd,nd+1).T[::-1]
         simplices = self.simplices[ielem, edges[iedge].T].T

tests/test_topology.py

@@ -1363,6 +1363,19 @@ def test_contiguous_simplices(self):
     def test_boundary(self):
         self.assertIsInstance(self.topo.boundary, topology.SimplexTopology)
 
+    def test_empty_boundary(self):
+        simplices = numpy.array([[0, 1, 2, 3]])
+        transforms = transformseq.IndexTransforms(3, len(simplices))
+        topo = topology.SimplexTopology('X', simplices, transforms, transforms)
+        self.assertEqual(len(topo.boundary), 4)
+        self.assertEqual(len(topo.boundary.boundary), 0)
+
+    def test_empty_interfaces(self):
+        simplices = numpy.array([[0, 1, 2, 3]])
+        transforms = transformseq.IndexTransforms(3, len(simplices))
+        topo = topology.SimplexTopology('X', simplices, transforms, transforms)
+        self.assertEqual(len(topo.interfaces), 0)
+
     def test_getitem(self):
         self.assertIsInstance(self.topo[numpy.arange(4) < 2], topology.SimplexTopology)
         self.assertIsInstance(self.topo[numpy.arange(2)], topology.SimplexTopology)