Stochastic Ebola model should have a two level structure

[pratikunterwegs]

This issue is to request that the Ebola model function model_ebola_r() be split into two functions to follow the organisation of the ODE deterministic models. This would allow input checks in the external user-facing function, while the internal 'unsafe' function would be run for $N$ replicates on parameter and event set combinations. See more on this in #160.