Move `preprocess_species.R` to BirdFlowPipeline

[chenyangkang]

preprocess_species.R is used for fitting models so does not seem to be BirdFlowR's job. Consider moving preprocess_species.R to BirdFlowPipeline. The preprocess_species function is using ebirdst package and ebirdst is listed as "suggested" installation without version control.

[ethanplunkett]

Lack of version control on ebirdst is deliberate to allow fitting any species. ebirdst/eBird 2022 only included half the species so users can choose to use ebirdst 2021 for the missing species. The North American species were all included in 2022 so this has not been very important to date. When they release 2023 data we may want to drop compatability with 2021 as it uses a different date format. See note in NEWS discussing the compatibility with both versions of ebirdst and help in get_dates() for a discussion of the different date formats.

ebirdst is in suggests rather than imports as I didn't think all users would need to run preprocess species - backing up your idea of moving it to BirdFlowPipeline.

I like the idea of rearranging things; the current organization is a product of the development history. It would be nice to keep preprocess_species() public though, so we might want to consider making BirdFlowPipeline public if we move preprocess_species() to it. #156 addresses this too, although when it was written BirdFlowPipeline was Dave's private repository so wasn't considered in the restructuring.