switch from np.product() to np.prod() as the former is deprecated

Problem & Solution Description (including issue #)

[ ] My code follows the code style of this project
[ ] I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
[ ] My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
[ ] Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings