Resolving issues #17, #34, #42

[wleoncio]

C++ code has been confirmed to be faster than R, which should be enough to close issue #17 (and, by extension, #42).

Unit tests have been added for all valid combinations of parallel and pal, which closes #34.

[wleoncio]

All CodeFactor issues are minor and most are not related to the issues mentioned here, so they should be worked on a different issue.

[wleoncio]

All checks failing on parallel code:

── Error ('test-parallel.R:78:3'): (code run outside of test_that()) ─────────
Error in .check_ncores(nnodes): 6 simultaneous processes spawned

Can reproduce locally, but only by running devtools::check(), as a manual run of the tests through devtools::test() does not show anything. I'm converting this PR to a draft while I investigate.

[wleoncio]

The solution might be simple and involve limiting the number of cores to 2 on the examples (source: https://stackoverflow.com/a/50571533/1169233).

[wleoncio]

Ready for review! Fixing the parallel test issues was as easy as setting cl = 2 (the GitHub Action and CRAN limits), and I also had to skip the parallel tests on Windows (will explain better on a separate issue).