Loosen signature of value and uncertainty for Unitful

[giordano]

Fix #87.

[codecov-io]

Codecov Report

Merging #88 (6fa1e8a) into master (1c1950e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   97.09%   97.09%           
=======================================
  Files          12       12           
  Lines         688      688           
=======================================
  Hits          668      668           
  Misses         20       20           

Impacted Files	Coverage Δ
src/unitful.jl	100.00% <100.00%> (ø)

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