various fixes

[danielnachun]

@pcarbo this is my first attempt at getting the new CI system working. In addition to adding the GitHub Actions pipeline, I also made a few small fixes to the package:

• Some URLs needed https instead of http, or needed a trailing slash
• The CITATION file needed some changes for newer versions of R
• Rcpp is not called in the R code, so it should only be included in LinkingTo and not Imports.

[danielnachun]

There are no warnings now, and only one note that actually needs to be fixed:

❯ checking Rd \usage sections ... WARNING
  Undocumented arguments in Rd file 'glm.approx.Rd'
    ‘eps’
  
  Undocumented arguments in Rd file 'smash.Rd'
    ‘...’
  
  Undocumented arguments in Rd file 'ti.thresh.Rd'
    ‘ashparam’
  
  Undocumented arguments in Rd file 'wls.mb.Rd'
    ‘z’ ‘v’ ‘disp’ ‘indexnm’ ‘ng’ ‘df’ ‘forcebin’ ‘g’ ‘n’ ‘vv’
  
  Functions with \usage entries need to have the appropriate \alias
  entries, and all their arguments documented.
  The \usage entries must correspond to syntactically valid R code.
  See chapter ‘Writing R documentation files’ in the ‘Writing R
  Extensions’ manual.

I believe this is just a matter of documenting arguments to these functions and then regenerating the documentation accordingly.