estimate_delays -> estimate_dist

[seabbs]

It would make sense to generalise and simplify the current functionality of estimate_delays into a new function called estimate_dist (to highlight it isn't just distributions we are estimating).

Based on lessons learnt in dynamictruncation we can now also do quite a bit better with a simple model and support right and potentially left truncation adjustment using the non-latent formulation.

This would only require some small adaption of the stan code with the majority of the work being to reduce the current 3 distribution model into one. This would also slightly reduce compilation times which I think everyone would like.

The argument to not making this model is that it can be trivially implemented in brms but I think many users may not realise this and there may be value in supplying a simple but still valid model to compete with the simple but not as valid alternatives.

[sbfnk]

An example of how this could be done in stan is in https://github.com/epiforecasts/marburg.parameters/blob/main/inst/stan/model.stan

[seabbs]

This is the current candidate model for implementation. In the first pass this would be constrained to daily events but should have better scaling with large amounts of data.