investigate crtp

[tbrown122387]

Investigate potential modest speed improvements resulting from using a curiously recurring template pattern instead of pure virtual functions in particle filter base classes.

This idea is largely motivated by this thread here: https://codereview.stackexchange.com/questions/236425/abstract-base-classes-versus-the-curiously-recurring-template-pattern?noredirect=1#comment463461_236425

[tbrown122387]

they would also help group different particle filtering algorithms into one base class. For example, if you use this as a guide, derived model-specific classes could define models' functions that take different numbers of parameters. This happens all the time in the state transition function, for example. It could take no arguments, one, two, etc.

This would most likely start out as a separate remote branch at first.