slab(): weighted variant of categorical method

[brownag]

Develop weighted variant of categorical method (created stub function .slab.fun.factor.weighted()).

This was carved out of #259 to keep that PR simpler and allow for more discussion about specifics of the method.

This method would be used in slab() by specifying slab(..., slab.fun =slab_function("weighted.factor") but this currently returns an empty function.