RcppEnsmallen 0.2.16.2.1 (GitHub-only Release)

• Upgraded to ensmallen 2.16.2: "Severely Dented Can Of Polyurethane" (2021-03-25)
  • Fix CNE test trials
    (#267).
  • Update Catch2 to 2.13.4
    (#268).
  • Fix typos in documentation
    (#270,
    #271).
  • Add clarifying comments in problems/ implementations
    (#276).
  • Remove AdamSchafferFunctionN2Test test from Adam test suite to prevent
    spurious issue on some aarch64 (#265).