export ud are convertible

[cregouby]

and fix #258

[Enchufa2]

Thanks, please see my comments.

[codecov]

Codecov Report

Merging #263 (84db29b) into master (3648766) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #263   +/-   ##
=======================================
  Coverage   94.27%   94.27%           
=======================================
  Files          18       18           
  Lines         873      873           
=======================================
  Hits          823      823           
  Misses         50       50           

Impacted Files	Coverage Δ
R/udunits.R	100.00% <100.00%> (ø)

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[Enchufa2]

Thanks!