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* PlanetModel renamed to HotPlanetModel
* Multivisit.fit_planet() renamed to Multivisit.fit_hotplanet()
* New PlanetModel that assumes flux from planet is due to reflection
* Multivisit.fit_planet() now assumes flux from planet is due to reflection
* Ensure funcs.eclipse_phase() returns phases in the range [0, 1)
* Change default keyword value a_c=0 to a_c=None so that it gets properly
initialised from the input datasets.