Deprecate fi_pool internals

[fredjaya]

          Another point this highlights is that perhaps ```fi_pool``` and ```fi_ratio``` are superfluous as they aren't really called by any user-facing functions. In particular ```fi_ratio``` is kind of just another way of viewing ```design_effect_cluster_fisher```. ```fi_pool``` is just a special case of ```fi_pool_imperfect```. Could we perhaps get rid of the two superfluous functions and then rename ```fi_pool_imperfect``` to ```fi_pool```?

Originally posted by @AngusMcLure in https://github.com/AngusMcLure/PoolPoweR/issues/6#issuecomment-1797163154

• [x] Add basic tests for fi_pool_imperfect
• [x] Remove fi_pool
• [x] Remove fi_ratio
• [x] Rename fi_pool_imperfect to fi_pool
• [x] Rename fi_pool_imperfect_cluster to fi_pool_cluster

[AngusMcLure]

The more I looked at the figure in #6, the more name changes I see.

I also suggest we change

• [x] unit_fi_cost_clustered to cost_fi_cluster
• [x] unit_fi_cost to cost_fi
• [x] design_effect_cluster_fisher to design_effect

[fredjaya]

Oops, would have broken a few optimise_prevalence.R functions when changing the fi_pool function names, which are unaccounted for in current tests.

Next, write basic unit tests for the following functions, mainly to catch any errors when renaming/updating:

• [ ] unit_fi_cost_clustered()
• [ ] design_effect_cluster_fisher()
• [ ] unit_cost_fi()

Rename fi_pool_imperfect to fi_pool in:

• [x] unit_fi_cost_clustered()
• [x] design_effect_cluster_fisher()
• [x] unit_cost_fi()

Then:

• [x] In design_effect_cluster_fisher(), rename fi_pool_imperfect_cluster() to fi_pool_cluster
• [x] Rename functions from the comment above

(messy but good enough for my reference)

[AngusMcLure]

I've used other IDEs that let you change functions signature across a whole project automatically. I don't think Rstudio has this, but Ctrl+Shift+F will probably let you find and replace the names throughout?

[fredjaya]

That should be all. Let me know if anything else should be amended, otherwise good to close the issue for now.