Remove extra imports in gamma.py doctests (#8060)

* Refactor bottom-up function to be class method

* Add type hints

* Update convolve function namespace

* Remove depreciated np.float

* updating DIRECTORY.md

* updating DIRECTORY.md

* updating DIRECTORY.md

* updating DIRECTORY.md

* Renamed function for consistency

* updating DIRECTORY.md

* Remove extra imports in gamma.py doctests

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Chris O <46587501+ChrisO345@users.noreply.github.com>