add transform.Point

This patch adds an implementation of `TransformItem` for points (`fromdims=0`).
This is seemingly useless, as the implementation doesn't add anything to its
base `Matrix`. However, the constructor is simpler than `Matrix` and `Matrix`
is currently used for points instances only. Replacing thoses instances with
`Point` is done in a follow-up patch.

nutils/transform.py

@@ -463,6 +463,12 @@ def __init__(self, trans1, trans2):
   def det(self):
     return self.trans1.det * self.trans2.det
 
+class Point(Matrix):
+
+  @types.apply_annotations
+  def __init__(self, offset:types.arraydata):
+    super().__init__(numpy.zeros((offset.shape[0],0)), offset)
+
 ## EVALUABLE TRANSFORM CHAIN
 
 class EvaluableTransformChain(Evaluable):

tests/test_transform.py

@@ -80,6 +80,11 @@ class SimplexChild(TestInvertible):
   def setUp(self):
     super().setUp(trans=transform.SimplexChild(3, 1), linear=numpy.eye(3)/2, offset=[.5,0,0])
 
+class Point(TestTransform):
+
+  def setUp(self):
+    super().setUp(trans=transform.Point(numpy.array([1.,2.,3.])), linear=numpy.zeros((3,0)), offset=[1.,2.,3.])
+
 del TestTransform, TestInvertible, TestUpdim