Adjust import statements to match convention

[Madddiiiin]

Description

The current situation in the core module is that there is a wild mix between two different approaches to importing functionality from libraries.

The first one is to use:
from numpy import power
...
power(x, y)

The second one is:
import numpy as np
...
np.power(x, y)

The team has decided to go for the second option and so it should be used everywhere.

Acceptance Criteria

• Every usage of the first option is replaced by the second
• No functionality is changed