Replace calls to numpy.product with numpy.prod

As of numpy 1.25.0, numpy.product is deprecated in favour of numpy.prod, see the release notes. It will be removed entirely in 2.0.0. Using it returns the following warning:

DeprecationWarning: `product` is deprecated as of NumPy 1.25.0, and will be removed in NumPy 2.0. Please use `prod` instead.

It is used in at least one place and referenced in some doc-strings. These should be updated to use prod.