implemented drop_front and drop_back with test.
drop_front and drop_back included in synopsis.
Merge branch 'feature/drop_front_back' into develop
fix for recently introduced regression to sub_range const propagation.
fix lifetime issue with test case for ticket 6715.
Merge branch 'develop'
These changes are just to address some small issues on some compiler versions with newly introduced tests. It also includes a fix to a recent regression in the sub_range class so that it preserves constness propagation as before the SFINAE improvement changes.