Fixed some minor things in the Correlation/Cov lab, inserted a section on
Covariance, added a problem, updated solutions file accordingly.
Added an additional comment to the corr/cov lab explaining the importance
of the covariance matrix.
Added section on LSI and information retrieval. Still need to write the
problem.
Merge pull request #717 from abefrandsen/covariance

Fixed some minor things in the Correlation/Cov lab, inserted a section o...