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:

GaussianFilters.jl/src/gmphd.jl

Line 22 in d264bd4

function update(phd::PHDFilter, b0::GaussianMixture,

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.