Test suites for vectorised ODE model

[pratikunterwegs]

This issue requests the addition of comprehensive testing suites for all ODE models with both scalar and vector or list-like values for the infection parameters and epidemic modifiers (interventions etc.).

Test must include:

• basic expectation of error free runs for scalar and list-like args;
• basic expectations on the output type and structure of runs with scalar arguments (see #177);
• basic expectations on the output type and structure of runs with list-like arguments (nested data.table);
• errors, warnings, and messages for runs with both scalar and list-like args;