Allow calling duration without times argument

[alecloudenback]

No description provided.

[codecov]

Codecov Report

Merging #35 (913df86) into master (05142d3) will decrease coverage by 2.16%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   95.49%   93.33%   -2.17%     
==========================================
  Files           3        3              
  Lines         111      120       +9     
==========================================
+ Hits          106      112       +6     
- Misses          5        8       +3     

Flag	Coverage Δ
unittests	93.33% <66.66%> (-2.17%)	⬇️

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Impacted Files	Coverage Δ
src/financial_math.jl	95.12% <66.66%> (-3.51%)	⬇️

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