Use delays_lp() in estimate_truncation

[seabbs]

Currently, estimate truncation is hard-coded to estimate a log normal whilst truncation inputs elsewhere are flexible. Package infra needs to be copied into estimate_truncation in order to standardise functionality.

[sbfnk]

Whilst doing so we could also update the model to be more like a simple version of the model in https://github.com/epinowcast/epinowcast, i.e. with an expectation model as well as a delay model. Or point people to epinowcast for truncation estimation (though would require people to install cmdstan and compile code).