Implement configurable and precise rounding

[mmomtchev]

This PR implements configurable and precise rounding of the results according to my proposal #128

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.91667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 98.55%. Comparing base (11162cb) to head (d95dede).

Files with missing lines	Patch %	Lines
src/geom.js	76.92%	3 Missing ⚠️
src/flight.js	75.00%	1 Missing ⚠️
src/scoring.js	99.34%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
- Coverage   98.60%   98.55%   -0.06%     
==========================================
  Files          10       10              
  Lines        2080     2211     +131     
==========================================
+ Hits         2051     2179     +128     
- Misses         29       32       +3     

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