don't defer negative binomial to Poisson at low levels of overdispersion

[sbfnk]

Existing comment in the stan code refers to avoiding overflows so will have to monitor that.

This also reinstates the tests for parameter recovery with the secondary model and fixes a bug where observation weights weren't used in the negative binomial model.

Closes #431
Addresses part of #429
Fixes #349

[seabbs]

LGTM. Version and news please otherwise good to go - depending on CI

[seabbs]

Can also drop this.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 0fbf93e is merged into main:

•   :ballot_box_with_check:default: 1.16m -> 1.12m [-18.84%, +12.4%]
•   :ballot_box_with_check:no_delays: 1.12m -> 1.13m [-22.98%, +25.12%]
•   :ballot_box_with_check:random_walk: 20.6s -> 21.6s [-15.44%, +24.87%]
•   :ballot_box_with_check:stationary: 39.7s -> 39.4s [-11.35%, +9.97%]
• ❗🐌uncertain: 1.46m -> 1.66m [+0.88%, +26.06%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 2833027 is merged into main:

•   :ballot_box_with_check:default: 49.7s -> 48.4s [-15.99%, +10.94%]
•   :ballot_box_with_check:no_delays: 49.4s -> 51.9s [-5.71%, +16.02%]
•   :ballot_box_with_check:random_walk: 15.4s -> 14.4s [-16.22%, +3.58%]
•   :ballot_box_with_check:stationary: 29.1s -> 29.8s [-9.16%, +13.88%]
•   :ballot_box_with_check:uncertain: 1.14m -> 1.07m [-17.1%, +5.44%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if aad2527 is merged into main:

•   :ballot_box_with_check:default: 53.3s -> 54s [-13.3%, +15.95%]
•   :ballot_box_with_check:no_delays: 55s -> 54.2s [-10.54%, +7.66%]
•   :ballot_box_with_check:random_walk: 15.7s -> 16s [-4.12%, +8.15%]
•   :ballot_box_with_check:stationary: 34.2s -> 33.1s [-16.13%, +9.36%]
•   :ballot_box_with_check:uncertain: 1.22m -> 1.25m [-12.44%, +17.96%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[sbfnk]

Two checks failing for apparently unrelated reasons.