Use a counter for the tag

[giordano]

Suggested by Steven G. Johnson. Fix #15.

[codecov-io]

Codecov Report

Merging #16 into master will increase coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   97.65%   97.85%   +0.19%     
==========================================
  Files           8        8              
  Lines         512      513       +1     
==========================================
+ Hits          500      502       +2     
+ Misses         12       11       -1

Impacted Files	Coverage Δ
src/derivatives-type.jl	100% <ø> (ø)	⬆️
src/math.jl	99.47% <100%> (ø)	⬆️
src/utils.jl	100% <100%> (ø)	⬆️
src/Measurements.jl	74.28% <100%> (+3.69%)	⬆️

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