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!

Great! thank you @meechan. I'd suggest a couple minor changes:

1. I think the preface text & code snippets, reading the images in with imread, would be better kept outside the exercise block, i.e.

## Reading data to a numpy array
We'll use the popular image analysis package scikit-image, to read two example images into numpy
arrays.
~~~
from skimage.io import imread
raw = imread('cilliated_cell.png')
nuclei = imread('cilliated_cell_nuclei.png')
# if you want to see what these images look like - we can use matplotlib (more to come later!)
import matplotlib.pyplot as plt
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?
[...]

2. Code blocks don't need to be blockquoted (> ), only wrapped in ~~~ - I've adjusted this in the example above.

Sounds good to me! I'll make those changes

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• 6a97b745 - move initial setup outside of exercise block

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Since writing that last comment, I learned that we need blank lines in the exercise blocks.

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• e268de35 - Apply suggestion to _episodes/02-data.md

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resolved all threads

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merged

mentioned in commit b64c9d1f