Create a `CustomEstimator` to ease interfacing with Low-level particle filter package

[franckgaga]

@baggepinnen your package LowLevelParticleFilter is starting to be really impressive in terms of really niche and cool features. It could be great to create a new StateEstimator type that eases the interface with it.

[1-Bart-1]

I have some ideas for a very specific estimator for a very specific use case as well, it would be cool if there would be a CustomEstimator so I can use my own estimators.