Fix #208: validate units before installation

[Enchufa2]

Current checks:

• Leading and trailing numbers.
• Arithmetic operators.
• Intermediate white spaces.

Am I missing something?

[codecov]

Codecov Report

Merging #209 into master will increase coverage by 0.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   94.23%   94.51%   +0.28%     
==========================================
  Files          18       18              
  Lines         919      930      +11     
==========================================
+ Hits          866      879      +13     
+ Misses         53       51       -2

Impacted Files	Coverage Δ
R/deprecated.R	83.33% <100%> (ø)	⬆️
R/make_units.R	94.31% <100%> (-0.07%)	⬇️
R/user_conversion.R	94.11% <100%> (+12.29%)	⬆️

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[edzer]

Looks good & makes a lot of sense - thanks!