+ some trivial indepCopula methods

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/nacopula@1073 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
claytonCopula(0) also returns indepCopula [by default]

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/nacopula@1074 edb9625f-4e0d-4859-8d74-9fd3b1da38cb