Optimise exponentiation

[GilesStrong]

https://chrissardegna.com/blog/posts/python-expontentiation-performance/ studies the performance of different methods of exponentiation and finds that chained multiplication should be used for integer powers less than, or equal to, 5, and math.pow() should be used otherwise. I.e. never use **.

It doesn't study exponentiation of Numpy arrays. Probably it will be useful to check if np.pow and ** are equivalent, and to compare math.pow to np.pow. There is also and argument for readability against chained multiplication.

This is only minor, but it could be useful to go through the code-base and optimise the exponentiation that is used. Probably just search for **.