Add numeric fallbacks for .value

[odow]

Closes #2260

cc @NLaws

[codecov]

Codecov Report

Merging #2262 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2262   +/-   ##
=======================================
  Coverage   91.37%   91.38%           
=======================================
  Files          42       42           
  Lines        4187     4189    +2     
=======================================
+ Hits         3826     3828    +2     
  Misses        361      361           

Impacted Files	Coverage Δ
src/variables.jl	96.13% <100.00%> (+0.03%)	⬆️

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