Djm pass CRAN checks

[dajmcdon]

Closes #19

The reason for doing this is that packages that take dspline as a dependency cannot be on CRAN unless this is.

There are a lot of file changes here, but they are mostly benign. Nothing touches the function of the package.

Major changes:

• All exported functions require @examples. I added very minimal ones, so I doubt they provide the user with much intuition, but they should satisfy checks. This basically meant that every file in R/ has changes. Other than the examples and a small change to the documentation for the dot functions, everything else (I think) is whitespace (annoying).

Minor changes:

• Adjust .Rbuildignore to avoid extra files.
• Create a new license (MIT, just updated)
• Use README.Rmd to create the README.md and move the figure code there (removing the old files).
• Add 2 Github actions, one that will run CRAN checks to preliminary ensure that we will pass on submission. The other will automatically build the website and run off gh-pages rather than a docs/ dir at the root.
• Adjust the language in the DESCRIPTION for CRAN compatibility. Reference your manuscript there. Also, clean up the dependencies a bit and add necessary fields for BugReports.
• Update the _pkgdown.yml to comply with newer `{pkgdown}.

Once we merge this, we basically just submit to CRAN which will trigger email to you.