Take unique() of add_vars, x, and y

[mayer79]

Small safeguard for sv_dependence2D(add_vars = ...)

[codecov-commenter]

Codecov Report

Merging #116 (61689f8) into main (4192d41) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 61689f8 differs from pull request most recent head 25eeb53. Consider uploading reports for the commit 25eeb53 to get more accurate results

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@           Coverage Diff           @@
##             main     #116   +/-   ##
=======================================
  Coverage   97.36%   97.36%           
=======================================
  Files          11       11           
  Lines         798      798           
=======================================
  Hits          777      777           
  Misses         21       21           

Files	Coverage Δ
R/sv_dependence2D.R	96.77% <100.00%> (ø)