install_local_single() supports a vector of subdirs, will look in each subdir for a DESCRIPTION file
and use the first directory where such a file is found (with warning in case of multiple matches)
by default, all install_* routines look in the root and in the "pkg" directory
for a DESCRIPTION file

fixes #323
documentation
update Rd files
update Rd files (unrelated)
Merge branch 'feature/r-forge-layout'