add intro numpy example

Basic intro exercise for numpy. The example images are in the repo under data/ @stamper @ralves @ext.bauer - happy for review from anyone!

_episodes/02-data.md

@@ -48,6 +48,7 @@ plt.imshow(raw, cmap='gray')
 plt.imshow(nuclei)
 ~~~
 {: .language-python }
+
 > ## Exploring Image Arrays
 > * What are the dimensions of these arrays?
 > * What data type are these arrays?