non-exact recycling in j results by group now supported, with one-time warning

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1161 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
all remaining use of memcpy size 1 now checked ok and commented

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/datatable@1162 edb9625f-4e0d-4859-8d74-9fd3b1da38cb