Revert Gibbs sampler back to sequential

[TommyJones]

Doing this b/c (a) doing parallel sampling in a way that respects the user's seed and conforms to CRAN policies was more trouble than it was worth because (b) parallel sampling (which was "approximate Gibbs" a-la Newman et al., 2009) significantly decreased goodness of fit by several metrics I usually rely on.

[codecov]

Codecov Report

Merging #53 (270cd08) into main (047ba67) will increase coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 270cd08 differs from pull request most recent head 265d950. Consider uploading reports for the commit 265d950 to get more accurate results

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   95.07%   95.13%   +0.06%     
==========================================
  Files          11       11              
  Lines        1117     1111       -6     
==========================================
- Hits         1062     1057       -5     
+ Misses         55       54       -1     

Impacted Files	Coverage Δ
R/utils.R	90.54% <100.00%> (+0.39%)	⬆️
src/lda_gibbs2.cpp	98.69% <100.00%> (-0.04%)	⬇️

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