I20 age structured seird model with NPIs

[oscardm20994]

Adds model which simulates non-pharmaceutical interventions to an age structured SEIRD model

[hfldai]

Checklist & log

• [x] Check all docstrings
• [x] + ode functions in docstrings
• [x] Allow arbitrary number of interventions
• [x] Examine time t in run() when time step is not integers
• [x] Vignette (sensitivity; compare with different interventions)
• [x] Transmission param beta_isolated - consider renaming? NPI may not only be isolation
• [x] Change model structure: NPI changes social contact matrices by some % of original, depending on different strategies of NPI. Some NPI also changes beta | Ref
• [x] Model renaming: SEIRDAge_interventions -> SEIRDAge_NPI
• [x] Allow different values for each transmission param for each age group
• [x] Include basic reproduction number
• [x] Complete vignette: 1) COVID19-related transmission param values; 2) R0 details; 3) Subtitles; 4) Review for models 5) Logistics & References
• [x] Complete unit test