add example showing how to make an elliptical grating

examples/ellip_grating.py

@@ -0,0 +1,22 @@
+# Quick script for testing arcs
+
+import numpy
+
+import masque
+from masque import shapes
+
+
+def main():
+    pat = masque.Pattern()
+    for rmin in numpy.arange(10, 15, 0.5):
+        pat.shapes.append(shapes.Arc(
+            radii=(rmin, rmin),
+            width=0.1,
+            angles=(-numpy.pi/4, numpy.pi/4)
+        ))
+
+    pat.visualize()
+
+
+if __name__ == '__main__':
+    main()