Refactor dedicated modules `model.py`

[cmp0xff]

          If we are moving initial conditions to a dedicated module, let's also move the following lines to a dedicated module (maybe `model.py` or something).

I feel most people won't look into __init__.py unless it is related to __all__.
Maybe we only do imports in __init__.py. Something like

from hamilflow.models.harmonic_oscillator.model import SimpleHarmonicOscillator, DampdHamonicOscillator


__all__ = ["SimpleHarmonicOscillator", "DampdHamonicOscillator"]

Originally posted by @emptymalei in #54 (comment)

[cmp0xff]

Do we still need a dedicated module model.py for all models, @emptymalei ? At the moment, only the complicated Kepler problem has submodules. All other models are contained in one file.