Add `np.savez` and `np.load` to Numpy episode

[ashwinvis]

Here: https://enccs.github.io/hpda-python/stack/#i-o-with-numpy

Good improvisation from @ffrancesco94 !

[ffrancesco94]

I will open a pull request with that today/tomorrow and we can merge it when we finish delivering it, considering we're probably making some changes in these days!

[ashwinvis]

Done in #98