Public repository for dotCall64 package

• CRAN package link: https://CRAN.R-project.org/package=dotCall64

• dotCall64 was by-product of the 64-bit extension of spam to spam64.
  For detailed examples, consult a recent source of the spam package. (E.g., https://cran.r-project.org/src/contrib/spam_2.8-0.tar.gz).
  The function spam::bandwidth() illustrates the R side of calling .C64().

• Yifan Liu has written a comprehensive and detailed tutorial, available at https://fortran-lang.discourse.group/t/extend-r-with-fortran/2386.