Make lobpcg::MagnitudeCorrection public and improve documentation

[bytesnake]

Document the lobpcg module, fixes#274:

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.18%. Comparing base (aee87d9) to head (451ed8e).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@ 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 

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