implement --before and --after meta parameters as alternative to require=""

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>
remove legacy code snippet: singleton object_ids have been removed in master

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>
--whitespace

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>
missing import

Signed-off-by: Steven Armstrong <steven@icarus.ethz.ch>