Remove Rcpp-specific approach and just use std::filesystem (or boost)

[bpbond]

See #692

[github-actions[bot]]

Differences in Hector outputs

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The current pull request's outputs do not differ from 3.0.0 (9aebf67).

[codecov-commenter]

Codecov Report

Merging #694 (ef1dc3f) into v3_dev (1ccacc9) will decrease coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           v3_dev     #694      +/-   ##
==========================================
- Coverage   79.69%   79.68%   -0.02%     
==========================================
  Files          61       61              
  Lines        5935     5932       -3     
==========================================
- Hits         4730     4727       -3     
  Misses       1205     1205              

Impacted Files	Coverage Δ
src/ini_to_core_reader.cpp	94.87% <ø> (-0.37%)	:arrow_down:

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