setactive

   NEWDIRSTRUCT = SETACTIVE(MYDIRSTRUCT, DIRLIST,APPEND)

   Sets the cell list of directories (or single character directory name)
   to be active in the DIRSTRUCT MYSDIRSTRUCT, and returns a new
   DIRSTRUCT.  If a directory does not exist, no error is given but the
   directory is not made active.  If append is 1, then these directories are
   appended to the list of active directories.  Otherwise, the list of
   active directories are set to be exactly DIRLIST and no others.