Remove use of isFALSE to allow use of R < 3.5

[langmm]

Closes #229

[codecov]

Codecov Report

Merging #230 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
+ Coverage   94.09%   94.09%   +<.01%     
==========================================
  Files          18       18              
  Lines         897      898       +1     
==========================================
+ Hits          844      845       +1     
  Misses         53       53

Impacted Files	Coverage Δ
R/symbolic_units.R	98.8% <100%> (+0.01%)	⬆️

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[Enchufa2]

I think it's more safe and clear if we just port the function and keep the syntax. Also, I've bumped the version and updated the NEWS. Many thanks!