Add init_likelihood arg

[SamDuffield]

Adds a compulsory init_likelihood argument to all init_prepare functions. It has default True so doesn't change any existing tests or code, although if we agree on this API we probably want to add tests for init_likelihood=False.

As discussed in #178 this is required for factorial models.

[SamDuffield]

Also I'm open to different options for the name of the arg, e.g. apply_update, apply_initial_observation ...?

[Sahel13]

I think apply_initial_observation, or a shorter apply_init_obs, is better.

[SamDuffield]

As mentioned in #184 we made the soft decision not to have this arg as it's cleaner to have a unified API controlled by model_inputs rather than adding args unnecessarily.

Soft decision is to refactor to remove initial observation everywhere.