Use log1p(x) instead of log(1+x)

[mllg]

log1p is numerically accurate for small x with 1 + x == 1:

x = 1e-20
log(1 + x)
log1p(x)

[codecov-io]

Codecov Report

Merging #39 into master will decrease coverage by 0.06%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage      98%   97.93%   -0.07%     
==========================================
  Files           5        5              
  Lines         100       97       -3     
==========================================
- Hits           98       95       -3     
  Misses          2        2

Impacted Files	Coverage Δ
R/regression.R	100% <100%> (ø)	:arrow_up:
R/information_retrieval.R	96.77% <0%> (-0.29%)	:arrow_down:

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