Refactor to type-based insurances instead of functional

[alecloudenback]

closes #40

[codecov]

Codecov Report

Merging #41 (a7ddf05) into master (70b2f71) will increase coverage by 0.56%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   89.51%   90.07%   +0.56%     
==========================================
  Files           2        2              
  Lines         143      131      -12     
==========================================
- Hits          128      118      -10     
+ Misses         15       13       -2     

Flag	Coverage Δ
unittests	90.07% <97.43%> (+0.56%)	⬆️

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Impacted Files	Coverage Δ
src/LifeContingencies.jl	96.72% <97.43%> (+1.19%)	⬆️

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[alecloudenback]

Would like to retain a kind of null or zero concept. E.g. an annuity with zero payments. How best to represent?
Ideas:

• return empty arrays
• a new struct that is an empty contract