Improving NumPy lecture to use the community-standard import

[john-science]

Throughout the numpy lecture and problem sets, I use imports like this:

https://github.com/john-science/python_for_scientists/blob/8d7a104f3bf282eadb43b3ab41af5fdcd2ce6d93/classes/10_numpy/lecture_10.md?plain=1#L33

But the community standard is to do this:

import numpy as np

np.array([1])
np.zeros(1)

The lecture should be revised to use the community-standard imports.

[john-science]

Fixed: https://github.com/john-science/python_for_scientists/commit/1eeaed6786d3f584bf631c7a7b9b7b11f1f7e6c8