File sizes

Encourage `@` operator over `np.dot`/`ndarray.dot`

This operator exists in all maintained versions of Python and numpy and produces more readable code.

Introduce in early numpy lesson, and replace dot() throughout when there is no reason to keep dot().

Replace numpy.random with `default_rng`

Move to the RandomGenerator API.

Refactor reshape_and_3d

Merge parts with reshaping section of Numpy intro