Explicit links to other packages in documentation

Added exlicit links to external functions:

• \code{\link{function}}: function in this package
• \code{\link[MASS]{abbey}}: function in another package
• \link[=dest]{name}: link to dest, but show name
• \code{\link[MASS:abbey]{name}}: link to function in another package, but show name
• \linkS4class{abc}: link to an S4 class