FEATURE(DiseasyActivity): Multiplicative application of activity reduction

[RasmusSkytte]

Presently, activity reductions are implicitly assumed to be "absolute" through the application of the "herringbone" matrix pattern when estimating contact matrices as restrictions change.

This feature requests suggest we add a multiplicative mode of accounting for activity reductions when constructing new contact matrices.

Overall I request the following features.

1. An argument to $initialize() that gives control the assumed method of applying activity reductions (e.g. "absolute" vs "multiplicative")
2. An active binding containing the chosen method (to ensure different module hashes)
3. Generalise $vector_to_matrix() with this same argument and perform an outer of the given vector if the argument is "multiplicative".