rewrite integrator

[mileslucas]

• switch logvol and logwt calculations to use quadratic trapezoid rule
• incorporate quad. trap. rule to logzerr calculation
• add quadratic trapezoidal rule to add_live_points
• rerun benchmarks with new integrator

[codecov]

Codecov Report

Merging #80 (5667ae0) into main (eb6999e) will decrease coverage by 0.16%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   88.62%   88.46%   -0.17%     
==========================================
  Files          13       11       -2     
  Lines         554      546       -8     
==========================================
- Hits          491      483       -8     
  Misses         63       63              

Impacted Files	Coverage Δ
src/step.jl	94.77% <96.66%> (+1.12%)	⬆️
src/proposals/Proposals.jl	91.25% <0.00%> (-1.74%)	⬇️
src/bounds/Bounds.jl	84.61% <0.00%> (-0.57%)	⬇️
src/bounds/ellipsoid.jl	85.22% <0.00%> (-0.17%)	⬇️
src/NestedSamplers.jl
src/models/Models.jl
src/bounds/multiellipsoid.jl	79.62% <0.00%> (+1.44%)	⬆️

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