Make compatible with CRAN again

[tretherington]

I received the following email on 23/05/2021, but haven't been able to address as yet, so virtualNicheR is no longer available on CRAN, and would be good to fix this when a moment presents itself:

Dear maintainer,

Please see the problems shown on
https://cran.r-project.org/web/checks/check_results_virtualNicheR.html.

Specifically, please see the problems for the Debian-based checks for r-devel.

These are from

  \item \code{matrix(x, n, m)} now warns in more cases where
  \code{length(x)} differs from than \code{n * m}; suggested by Abby
  Spurdle and Wolfgang Huber in Feb 2021 on the R-devel mailing
  list.

  This warning can be turned into an error by setting environment
  variable \env{_R_CHECK_MATRIX_DATA_} to \samp{TRUE}: \command{R
    CMD check --as-cran} does so unless it is already set.

in current r-devel, where the Debian-based checks are now performed with R_CHECK_MATRIX_DATA=TRUE.

Please correct before 2021-06-05 to safely retain your package on CRAN.

Best,
-k