Allow vectors in and out of rate matrix expressions

[stevencarlislewalker]

Story

As a pandemic modeller, I want to be able to get the speed benefits from TMB for models with structure, so that I can try out more models.

Note: Starting with vaxination as the test case.

Tasks

• [x] Symbolic manipulation of matrices and vectors
• [x] Make it possible to assign scalar expressions to multiple rates
• [x] allow character and struc 'formulas'
• [x] express foi as model structure
• [x] Make it possible to assign vector expressions to multiple rates