Fixed a problem with the CorrelationMatrix(PvlObject data) constructor giving...

Fixed a problem with the CorrelationMatrix(PvlObject data) constructor giving a segmentation fault when run in the unit test. Also updated documentation for many of the functions.  Fixes #3999.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/branches/ipce@6764 41f8697f-d340-4b68-9986-7bafba869bb8