Simplify linear index handling

[astamm]

Directly compute indices rather than using linear index table. Fixes Integer overlow in linear_index for semi-large data sets. Fixes #17.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 32.11679% with 93 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.46%. Comparing base (a1f904a) to head (7cfffb9).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
R/utils.R	24.39%	93 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   76.95%   75.46%   -1.49%     
==========================================
  Files          38       38              
  Lines        2256     2299      +43     
==========================================
- Hits         1736     1735       -1     
- Misses        520      564      +44     

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