RcppEnsmallen 0.2.18.0.1

• Upgraded to ensmallen 2.18.0: "Fairmount Bagel" (2021-10-21)
  • Add gradient value clipping and gradient norm scaling callback
    (mlpack/ensmallen#315).
  • Remove superfluous CMake option to build the tests
    (mlpack/ensmallen#313).
  • Bump minimum Armadillo version to 9.800
    (mlpack/ensmallen#318).
  • Update Catch2 to 2.13.7
    (mlpack/ensmallen#322).
  • Remove redundant template argument for C++20 compatibility
    (mlpack/ensmallen#324).
  • Fix MOEAD test stability
    (mlpack/ensmallen#327).
• Update GitHub Actions to the standard r-lib/actions configuration.
  (#45)