Leaky abstraction of PHDFilter

[lassepe]

The PHDFilter currently seems a bit out of place. It is not an AbstractFiltert and the update dispatch expects s a different signature: https://github.com/sisl/GaussianFilters.jl/blob/d264bd43337b77864c1756d83665f1af871c93fc/src/gmphd.jl#L22 That is, it does not have the usual update(filter, belief, action, observation) layout and thus all the functionality of src/simulate.jl simply does not apply. It would be nice to streamline this, or remove the PHDFilter from this package, or find a useful abstraction for it.