add continuous boyce index

[tiemvanderdeure]

closes #51

Todo:

• add tests
• move WARN_UNORDERED to somewhere more discoverable

[codecov]

Codecov Report

❌ Patch coverage is 7.69231% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.23%. Comparing base (2fa9c55) to head (9b55804).
⚠️ Report is 11 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/probabilistic.jl	7.69%	36 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #52      +/-   ##
==========================================
- Coverage   91.87%   87.23%   -4.65%     
==========================================
  Files          14       14              
  Lines         714      752      +38     
==========================================
  Hits          656      656              
- Misses         58       96      +38     

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