Add options to change galactic orbit integrator

[jbirky]

added options to change the integrator, with defaults:

integrator=gi.DOPRI853Integrator 
integrator_kwargs={"a_tol": 1e-10, "r_tol": 1e-10}

[TomWagg]

Love this @jbirky! Just a couple of small things and then I'll merge this:

• Please update cogsworth/_version.py and docs/modules/changelog.rst to reflect the changes (I think this is worth a bump to v2.1.1)
• It would be nice to save this saved to the Population on file as well. Could you update Population.save() and cogsworth.pop.load()?
  • You can copy what sfh_params does for saving the integrator params
  • For saving the Integrator, you can save the class name as a string and then use getattr(gi, NAME) to get it back for cogsworth.pop.load()

[TomWagg]

@jbirky actually do v2.1.2 sorry, I just merged a PR (#157) which did v2.1.1