refactor: use flatter import

src/psimpy/inference/active_learning.py

src/psimpy/inference/bayes_inference.py

src/psimpy/sampler/latin.py

src/psimpy/sampler/metropolis_hastings.py

src/psimpy/sampler/saltelli.py

src/psimpy/simulator/mass_point_model.py

src/psimpy/simulator/ravaflow24.py