Make lobpcg::MagnitudeCorrection public and improve documentation

[bytesnake]

Document the lobpcg module, fixes #274:

• [x] Make MagnitudeCorrection public
• [x] Improve documentation for TruncatedEig
• [x] Improve documentation for TruncatedSvd
• [x] Add Marchensko-Pastur law test

[codecov[bot]]

Codecov Report

Merging #275 (451ed8e) into master (aee87d9) will increase coverage by 0.23%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   88.95%   89.18%   +0.23%     
==========================================
  Files          71       71              
  Lines        3557     3578      +21     
==========================================
+ Hits         3164     3191      +27     
+ Misses        393      387       -6     

Impacted Files	Coverage Δ
ndarray-linalg/src/lobpcg/eig.rs	75.38% <ø> (ø)
ndarray-linalg/src/lobpcg/lobpcg.rs	93.30% <ø> (+1.18%)	:arrow_up:
ndarray-linalg/src/lobpcg/svd.rs	91.26% <100.00%> (+5.89%)	:arrow_up:

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