Refactor argument checker functions to cross-checking only

[pratikunterwegs]

This issue is to request a change to the model argument checker and preparation functions, .check_args_model_*() and .prepare_args_model_default(), to operate only on a list of the composable model elements in each model call: the population, intervention, vaccination, time_dependence and (for the diphtheria model only) population_change.

Previously, these functions were also used to prepare the infection parameters (e.g. conversion from $R_0$ to $\beta$), but this is no longer the case as of #140.

The functions need not be vectorised.