avoid import numpy::* in examples

[droundy]

This makes it very hard to read and understand the code. I can understand (but don't like) the use pyo3::prelude::*;, but having two wildcard imports makes it impossible to guess what any given symbol might mean.

[kngwyu]

Thanks for reporting!

having two wildcard imports makes it impossible to guess what any given symbol might mean

This sounds reasonable. I'm going to fix it.