S4 tweaks

o  revised documentation for all classes and methods
o  minor (mostly cosmetic) changes to the underlying code
o  new initialize methods for "CovarianceMatrix" and "Variance"
o  a few more classes and methods exported (for consistency)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/coin@897 edb9625f-4e0d-4859-8d74-9fd3b1da38cb