Test the synthesized 'product model' framework from #207 against classic macpan usecases

[stevencarlislewalker]

verify that existing flexmodel rate specification frameworks allow for more than a 'few' usecases

• [x] rep_rate
• [x] vec_rate without fancy matrix algebra (e.g. parameters vary among sub-models)
• [x] vec_rate with fancy matrix algebra (e.g. force of infection among multiple vaccination status categories)

verify all current macpan usecases

• [x] age-structure
• [x] vaccination
• [ ] variants
• [x] testify
• [ ] accumulators

[stevencarlislewalker]

no longer relevant because it is superseded by the graph theory paper -- #234