Add the initial mass function scripts

.gitlab-ci.yml

examples/initial_mass_function/plot_initial_mass_function.py

examples/initial_mass_function/run.sh

pyswiftsim/__init__.py

scripts/pyswift_initial_mass_function

setup.py

src/cooling_wrapper.c

src/libstellar.c

src/stellar_evolution_wrapper.c

tests/run_tests.sh

tests/test_initial_mass_function.py