add initial Dimension class

Dimension.py

@@ -0,0 +1,10 @@
+
+class Dimension(object):
+    def __init__(self, ptref1, ptref2, dist_con, offset):
+        self.ptref1 = ptref1
+        self.ptref2 = ptref2
+        self.dist_con = dist_con
+        self.offset = offset
+
+    def set_distance(self, d):
+        self.dist_con.dist = d