Adding parallel, filtering by county, fixing CI, and R CMD check fixes

[1beb]

1. Added parallel processing census_geo_api to expedite the process of connecting with the API via the get_census_api function. The implementation is setup such that there will be a county-per-thread model. Ostensibly, a user would specify their own plan, for example: library(future); plan(multisession). This has significantly decreased the total time to run the process. Currently uses furrr::future_map_dfr with a progress indicator. Reduces the processing time for a state from more than a day to less than an hour.
2. Converted print to message so that it follows stdin/out convention.
3. Setting default retries to 3. This avoids issues where minor internet instability requires a process restart.
4. Additional log messaging to identify the start of a long process.
5. Updated description to alphabetize imports, add future, furr and purr.
6. Updated roxygen docs.
7. Adding wru.png to .Rbuildignore to relagate an R CMD check note.
8. Integrated changes with hwru branch
9. Implemented piggyback for last_c, mid_c, and first_c files, temporarily using private repo "solivella/wruData"
10. Adding "use_counties = TRUE" flag such that census data is captured only for those counties that appear in voter.file data.
11. Added functionality for reading CENSUS_API_KEY from env var, to align with tidycensus. Updated readme with instructions.
12. Fixes some issues with undocumented or missing arguments for a number of functions
13. Fixes tests
14. Fixes CI

[1beb]

Notes:

• In sample data (data(voters)) when running by block with surname = F
• In new branch, data(voters) includes a duplicate column named "last" where "surname" already exists. "first" was also added.
• When using the new use_counties flag, there is an unexpected variation in results which I noted in slack.
• 12x speed up using use_counties flag.
• Tests are passing locally (with CENSUS_API_KEY in .Rprofile). However, two probabilities have changed and need to be verified.

[1beb]

@solivella ready for review.

[solivella]

Awesome. Thank you, @1beb!

[1beb]

@solivella here.